hmm, with 3 boxing you got pros and cons. This is what i think :P

3box:
Pros:
-Group bonus exp.
-Better to control.
-Cheaper to play.
Cons:
-Very hard to solo dungeons (almost inpossible).

5box:
Pros:
-You can dungeon farm for better exp.
-Cool formations. (Possible with 3box aswell, but then you will have less chars in the formations).
Cons:
Costs alot (think about 5 accounts montly fee).