If you're asking about changing from 4/4 to 3/4 in the middle of an arrangement...I don't believe it can be done in MMM.

However, to set up an arrangement for 3/4 time, go to....

File (menu) > Settings > Arrangement Settings... (E) > General (tab) > Beat Type (3/4)

Be aware however that this would likely be useful only for completely original compositions.  I believe the vast majority (if not all) of soundpool samples are in 4/4 time.