Number of bars/measures per line

[dflat]

Is there a way to have any number of bars/measures per line? iRealBook seems to force 4 measures per line no matter what.

No there are only 16 fixed-width spaces available per system (across) which cannot be altered. They accommodate a standard iReal Book width of a chord and its quality - like F7b9 etc.
For what you are doing, you could either make a blank space at the end of each 3 bar set (where the extra bar would be) ... or move the barlines so you only have 3 bars across a system (one bar having an extra beat - 3 bars x 5 spaces = 15 (+1 left over = 16) which is presumably what you have tried.
[in iReal book's Song edit mode: Symbols>Insert beat]

Something like this:
3 Per System by Unknown Composer

There was a reference here also:
http://irealb.com/forums/showthread.php?t=500

Thanks for your question.