To clarify the issue in question, i.e. permissibility of performing the obligatory daily prayers in three times, and the significance of combining or not combining the prayers, it is necessary that the following two elements of the subject-matter be investigated separately:
1. Permissibility of performing the prayers in three times: When it comes to permissibility of combining the prayers or performing the prayers in three times, a lot of discussions have taken place with Sunnis objecting to Shiites telling them that it is not permissible to combine the prayers and that such a practice is subject to criticism. On the contrary, Shiites are of the view that there is no problem in combining the prayers as there are narrations which support this practice.  For instance, Imam Sadiq (AS) was asked about the timing of the Zuhr and Asr prayers and he answered: "When the sun crosses the meridan (when sun reaches its highest point - nearest zenith) and begins to decline, then it is the time for both Zuhr and Asr prayers with the only difference that Zuhr prayer should be performed before the Asr prayer. Thereupon, the time will be for both prayers until the sun sets."[1]
At this stage, we can prove the point by presenting only one of many arguments. Based on the discussions that have taken place, it is inferred that most Muslim scholars maintain that the permissibility of combining prayers is substantiated and proved.
2. The significance of performing prayers in three or five times: Most Shia and Sunni scholars say that performing the prayers in five separate times is better and more significant. Shiite jurisprudents have explicitly stated this and they have dedicated a section in their books of Islamic Laws (Tawdhih al-Masail). For example, it is mentioned therein as such: It is recommended to perform the five prayers in five separate times; that is, each prayer should be performed in its respective time of fadhilah (significance).[2]
This rule is evidenced by narrations which denote the significance of praying in five times:
Imam Sadiq (AS) says:  "Verily, the Holy Prophet (S) combined Maghrib and Isha prayers and Zuhr and Asr prayers and he was doing so when he was in a hurry telling others that it was better to perform these prayers separately."[3]
Therefore, it has been clarified that Shiites relying on the narrations which allow the prayers to be combined do not seek to state that it is also more significant to combine them; in fact, they believe only in the permissibility of doing so. On the contrary, they accept that performing the prayers separately is better and does earn the individual more reward.
Now, a question arises as to why Shiites are practically combining their prayers despite the fact that performing the prayers separately is more preferred. There is an injunction in the Quran that virtue and excellence is to stay awake and engage in worshipping two third or half or one third of the night: "Surely your Lord knows that you pass in prayer nearly two-thirds of the night, and (sometimes) half of it, and (sometimes) a third of it, and (also) a party of those with you; and Allah measures the night and the day. He knows that you are not able to do it, so He has turned to you (mercifully), therefore read what is easy of the Quran… that is best and greatest in reward; and ask forgiveness of Allah; surely Allah is Forgiving, Merciful."[4]
It is inferred from this verse and other verses of the Quran clearly that staying awake and engaging in acts of worship have significant virtues and rewards and that constitutes part of the Shia and Sunni beliefs. However, the verse does mention difficulties which believers may confront in prayer but it finally gives concession which extends to believers by saying: "Recite, then, of the Qur'an that which is easy for you."[5] That is to say, do as much of it as you can. It is also observed among Muslims that due, mainly, to difficulties in life, they cannot engage in worshipping throughout most of the night and also be busy working throughout the day. In fact, they perform the prayers separately as far as they can but they will not be punished for not realizing all the rewards.
The subject in question is also of the same nature. Performing the prayers in five separate times is recommended and preferred, but it is not easy for them to perform them separately in view of the difficulties and problems overshadowing public lives. That is why, they believe it is permissible to combine the prayers and perform them in three times and there is also a tradition from the Holy Prophet (S) who said that he wanted that no one among his Ummah should be put to (unnecessary) hardship.”[6] Yet still, performing the prayers in five separate times is preferred and, if possible, one should perform each of the obligatory daily in their respective time of significance. This truth is evidenced by the fact that there are some mosques in Shia community where the five daily prayers are offered in five separate times so that more rewards and virtue may be gained.

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