crank set woes

[ditz]

I pulled a diamond frame out of a trash compactor to save for commuting. I need some low gearing for the hills in my area so I like an idiot went rummaging for a cheap mtn bike to rob the triple chain ring crank set and gear changers to put on the diamond frame. Well you fellas know where this is heading. The bottom bracket of the mtn bike is a larger diameter than the diamond frame bottom bracket. The crank sets do not appear to be interchangable. Is there a cheap fix for this problem? ..........thanks for your comments