Should somebody put some real information here?
3 x 10's --> ~235 sq in.
3 x 12's --> ~339 sq in.
2 x 15's --> ~353 sq in.
These are the approximate cone areas of the three different options...under the assumption that 100% of the width is cone. Now, this is not a good assumption, but assuming you're speaking of 3 setups using the same make, just different sizes, then the comparison and the size ratio is valid.
Cone area is not everything though...volume of air moved is. This factors in the linear motion of the sub through a cycle, which depends on the power, the box type, etc.
The real question you should be asking is...what size enclosure can I fit, and what size enclosure does each sub require? Then figure out what you want.
In general though, I'd say the 15's...they're usually not much more expensive than the 12's, and you only need 2 rather than 3. This means less money on drivers, less power needed, etc. Plus...in theory, they should be louder anyways.
To the man who said get a single 15 and be done though, tis true.