My son is in a built up triple. After “meeting” the roommates via text and talking for a few weeks he approached the subject of the beds. He asked if, because he lives the farthest away (much farther) than the other two, and will not be able to go home much, AND because he is 6’3" tall (can not see himself on top bunk with his size) could he have the single bed (non-bunk). There are only 2 closets in the room. He offered that the other two roommates could have the closets and he would bring just a couple of things to hang up. So essentially he gave up a closet for a single bed. The roommates were very kind and agreed. So far, all three roommates have been very flexible and agreeable.
Both my kids talked with roommates prior to move in about arrangements but realistically they decided together once they got there but both had doubles (son had a triple but just 2 kids… Very lucky…). Both lofted.
Make sure you get a safety rail if you are on the top bunk/loft:
http://talk.qa.collegeconfidential.com/parents-forum/1911839-bunk-and-loft-bed-dangers.html#latest