Why are three "Comings" a problem? I have been reading up on various eschatological views and I have noticed an argument that I have not been able to find a detail about. It seems to be a common assertion by the Catholic and Orthodox that the Rapture, as portrayed by the Left Behind series, is non biblical because it calls for three "comings" of Jesus. 

catholic.org puts it eloquently enough,


  The problem with all of the positions (except the historic, post-tribulational view, which was accepted by all Christians, including non-premillennialists) is that they split the Second Coming into different events. In the case of the pre-trib view, Christ is thought to have three comings—one when he was born in Bethlehem, one when he returns for the rapture at the tribulation’s beginning, and one at tribulation’s end, when he establishes the millennium. This three-comings view is foreign to Scripture. 


Does the bible specifically limit the number of times that Jesus comes to earth? Why would it be a problem for Jesus to come again before the "Second Coming"? Where does this "only two comings" requirement come from?
The need for three comings presupposes the following:


there will be a future period characterized by great suffering referred to as the Great Tribulation
the primary purpose of the secret preliminary coming is to take Christians away from the earth to prevent their experiencing this tribulation


Some non-rapturists, preterists specifically, see these presuppositions as problematic.  The biblical passages commonly used by rapturists to support these ideas have other interpretations that do not require a third coming and may have more evidence in their favor.  For instance, most preterists understand the fulfillment of passages like Matthew 24:36-45, Daniel 9:12,12:1, and Revelation 7:14 as the descriptions of the persecution of the Jewish people under Antiochus Epiphanes and later, the destruction of Jerusalem in 70 A.D. by the Romans.

In recent years, fewer rapturists are claiming support for the rapture in Matthew 24:36-45.  A few who still do are John F. Hart, professor at Moody Bible Institute and author Dave Hunt.

Dave Hunt states, in "How Close Are We? Compelling Evidence for the Soon Return of Christ", pages 210-11, that:


  When Christ says, "As it was in the days of Noah and Lot," it is absolutely certain that He is not describing conditions that will prevail at the time of the Second Coming. Therefore, these must be the conditions which will prevail just prior to the Rapture at a different time—and, obviously, before the devastation of the tribulation period.


John F. Hart has written the following 4 articles where he defends the idea that the rapture is referenced in Matthew:
A Defense of the Pretribulational Rapture in Matthew 24:36–44
Should Pretribulationists Reconsider the Rapture in Matthew 24:36-44? - Part 1
Should Pretribulationists Reconsider the Rapture in Matthew 24:36-44? - Part 2
Should Pretribulationists Reconsider the Rapture in Matthew 24:36-44? - Part 3  

Partial preterist Steve Gregg attempts to refute the above interpretation of Matthew 24:36-45 in his article Who's Been Left Behind

He essentially argues as follows:  

One rapturist interpretation of these verses supposes that the people taken are being saved from the tribulation and those who are left will suffer through it.  However, who are "they" who are "taken" in verse 39? "Noah" and his family or the other "people"? Grammatically, "they" must be the other "people" because "only the days of Noah" is specified, not his family.  And are "they" taken for judgement or redemption? The "people" who were taken in Noah's day were taken as a form of judgement. See Genesis 7:21,23.

However, many rapturists in fact reject the idea that Matthew 24:36-45 describes the rapture.  Although, they still believe it refers to a future Great Tribulation and the final phase of the future Second coming.

Even this view is troublesome for some non-rapturists, preterists specifically.  They see Luke 21:20-24 as a passage directly parallel to Matthew 24:36-45.  Most rapturists, like most scholars, concede that Luke 21:20-24 actually refers to the events of 70 A.D.  One such rapturist is Thomas Ice.

Here's how one preterist, Parker, argues that Ice's acceptance of Luke 21:20-24 as having been fulfilled in 70 A.D. poses a significant difficulty with regard to his contention that Matthew 24:15-21 is a reference to the last phase of the final coming of Christ following the Great Tribulation.  Parker believes that a verse-by-verse comparison of Matthew 24 to Luke 21 demonstrates that although a few statements are radically different between the two and seem to describe events that have not yet occurred, the differences can be accounted for in terms of symbolic language as used in the Old Testament.  Consequently, he reason's that if Luke 21:20-24 is fulfilled, the coming of Christ and tribulation of Matthew 24:15-21 must be as well.  And if the Great Tribulation is past, a two-phase final coming of Christ would seem unnecessary.

For reference, in the table of "Rapture &amp; Second Coming Passages" found on page 3 of Differences Between the Rapture and the Second Coming , Ice excludes Luke 21:20-24 from both the Rapture and Second Coming columns.  Matthew 24:15-21 however is included in the Second Coming column.

Also, in "Differences" above, Ice states:


  Many important biblical doctrines are not given to us directly from a single verse, we often need to harmonize passages into systematic conclusions. Some truths are directly stated in the Bible, such as the deity of Christ (John 1:1, Titus 2:13). But doctrines like the Trinity and the incarnate nature of Christ are the product of biblical harmonization. Taking into account all biblical texts, orthodox theologians, over time, recognized that God is a Trinity and that Christ is the God-Man. Similarly, a systematic consideration of all biblical passages reveals that Scripture teaches two future comings."


Non-rapturists tend to see the exegetical techniques through which  rapturists harmonize passages into conclusions as being very unsystematic.  

These techniques are argued to be vague and/or inconsistent for determining when:


dissimilarity in descriptions between particular passages demonstrates distinct referents
similarity in descriptions between particular passages demonstrates identical referents


Partial-preterist Hank Hanegraaff criticizes pretribulationist Tim LaHaye's definition of literalism on page 16 of his book "Apocalypse Code" as follows:


  "Not only is there nothing distinctive about this definition, but it is so vague as to be utterly useless."


Non-rapturists typically also express risks that the above mentioned exegetical techniques can result in dangerous interpretations of passages that they believe to be foundational to essential Christian beliefs and/or conduct consistent with those beliefs.