3 inch wheel is way too big.
You should probably ask some questions:
1) under load, what will be the approximate RPM of the motor?
2) what is the speed necessary reach the top in under 2 seconds?
3) what is the diameter of the wheel needed to reach that speed?
Those will get you in the ballpark.
You can get more precise by:
1) trying different tests
2) using explanations of the math/physics- there may even be some spreadsheets you can use:
http://www.chiefdelphi.com/forums/sh...threadid=93782
There are several variables that need to be controlled to make a direct drive minibot work.