Praise be to Allaah.
There is nothing wrong with delaying the marriage contract, even if the engagement lasts for a long time. There is nothing in sharee'ah to indicate the length of time that there should be between the proposal and the marriage contract, rather that depends on local customs and how prepared each party is to go ahead with the marriage contract. If a man may propose then do the marriage contract and consummate the marriage contract all in one day, or it may be done in a month or a year or longer than that.
But it is better - and we advise you - not to let the engagement go on too long, so long as the man is able to complete the marriage contract, because of the reports which encourage the one who can afford it to get married. The Prophet (peace and blessings of Allaah be upon him) said: O young men, whoever among you can afford it, let him get married, and whoever cannot afford it should fast, for it will be a shield for him. Narrated by al-Bukhaari (5065), Muslim (1400).
A great deal of experience shows that a lengthy engagement period opens the door to problems for both parties before they have really started their life together. These problems often lead to the cancellation of the engagement or they have long-lasting effects on the psyches of both parties.
What we advise you - the suitor and the family of the girl - to do, if the agreed upon engagement period is three years, as stated in the question, is not to hasten to do the marriage contract, because there is in fact no benefit in doing the marriage contract so early, if both parties realize that the man is a stranger (non-mahram) to his fiancÃ©e like any other stranger, until the marriage contract is done, and they are serious in adhering to the rulings and etiquette of that. What makes us offer this advice to you is what we have seen of many problems arising from allowing a lengthy time between the marriage contract and consummation of the marriage, some of which ended in the cancellation of the marriage. No doubt cancellation of an engagement is easier on both parties than the annulment of a shar'i marriage contract.
Moreover, another negative effect of a lengthy period between the marriage contract and consummation of the marriage is the fact that the two parties become more attached to one another, and their hearts and minds become distracted for no reason, which may affect them psychologically and distract them from the purpose for which they were created, such as acquiring beneficial knowledge and doing righteous deeds.
Think about the confirmation of that in the amazing story that the Prophet (peace and blessings of Allaah be upon him) told in order to teach a lesson, as it is narrated in Saheeh al-Bukhaari (3214) and Saheeh Muslim (1747) that Abu Hurayrah (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: One of the Prophets went out on a campaign, and he said to his people: No man should accompany me who has gotten married and wants to consummate the marriage but has not yet done so, or a man who has built a house but has not yet put the roof on, or a man who has bought some sheep or pregnant she-camels and is waiting for them to give birthâ€¦ This is well known.
The point here is that this noble Prophet excluded from the important mission of jihad those who were not fit for it, among whom was a man who had married a woman, and wanted to consummate the marriage, but he had not yet fulfilled that wish.
Ibn Battaal narrated that al-Muhallab, one of the commentators of al-Bukhaari, said: This indicates that the temptations of this world cause one to become cowardly and may lead to one's downfall, because whoever has gotten married to a woman but has not consummated the marriage with her, or he consummated the marriage only recently, his heart will be longing to go back to her, and the shaytaan will distract him from the worship that he is doing, so he will make him anxious. This applies to all worldly pleasures and possessions.
Al-Haafiz ibn Hajar (may Allaah have mercy on him) said: The point here is that one should be focused on jihad, because whoever enters into a marriage contract with a woman, his mind will be preoccupied with her, unlike the case if he has consummated the marriage with her, because in that case the matter is less serious in most cases. This is eating before praying.
But the advice given above only applies if you are unable to get married straight away, for some pressing reason. But we think that delaying it on the basis of studying is not a sound reason, and we do not advise that.
Shaykh Abd al-Azeez ibn Baaz (may Allaah have mercy on him) said:
What is required is to hasten to get married, and no young man or young woman should delay marriage for the sake of studies, because marriage does not prevent any such thing. It is possible for a young man to get married in order to protect his religious commitment and morals, and enable him to lower his gaze. Marriage serves many purposes, especially in this day and age. Because delaying it is harmful for both young women and young men, every young man and every young woman should hasten to get married if there is a man who is compatible with the woman, and if a man can find the right woman - in accordance with the words of the Prophet (peace and blessings of Allaah be upon him): O young men, whoever among you can afford it, let him get married, and whoever cannot afford it should fast, for it will be a shield for him. Saheeh - agreed upon.
This includes both young men and young women; it does not apply only to men, rather it includes everyone - they all need to get married. We ask Allaah to guide us all.
Majmoo'Fataawa wa Maqaalaat Mutanawwi'ah, 20/421-422.
Shaykh Ibn Uthaymeen (may Allaah have mercy on him) was asked:
There is a widespread habit whereby a girl or her father will refuse marriage to one who proposes to her so that she may complete her secondary or university education, or so that she may teach for a number of years. What is the ruling on that, and what is your advice to the one who does that? Some girls may reach the age of thirty and not be married.
The ruling on that is that it is contrary to the command of the Prophet (peace and blessings of Allaah be upon him), because the Prophet (peace and blessings of Allaah be upon him) said: If there comes to you one with whose character and religious commitment you are pleased, then marry (your daughter or female relative under your care) to him. (al-Tirmidhi, 1084). And he said: O young men, whoever among you can afford it, let him get married, for it is more effective in lowering the gaze and protecting one's chastity.
Refusal to get married means losing out on the benefits of marriage. What I advise my Muslim brothers who are guardians of women, and my Muslim sisters, to do is not to refuse marriage for the sake of completing studies or teaching. A woman can stipulate that her husband let her stay in school until her studies have ended, or that he let her continue to work as a teacher for one or two years, so long as she is not busy with her children. There is nothing wrong with that. Moreover, the importance of a woman acquiring university-level knowledge in a field that we do not need is something that may be subject to further examination.
What I think is that if a woman has completed the elementary level of education, and she knows how to read and write to such a level that she can benefit from reading the Book of Allaah and its tafseer, and reading the ahaadeeth of the Prophet (peace and blessings of Allaah be upon him) and its commentary, then that is sufficient, unless she wants to study fields of knowledge that the people need, such as medicine and the like, so long as there is nothing haraam in her studies and it does not involve mixing and the like.
Fataawa Ulama' al-Balad al-Haraam (p. 390).
May Allaah help us and you to do that which He loves and which pleases Him.
And Allaah knows best.