first order math:
the speed is # revolutions * circumference of wheel.
circumference = PI * R^2 /4
so the 3 inch wheel will be 9/4 == 2.25 as fast as the 2 inch wheel.
update:
circumference = PI * R / 2
so the 3 inch wheel will be 3/2 == 1.5 as fast as the 2 inch wheel.
second order math:
The 3 inch wheel is a bit heavier so it takes a bit more energy to move. (ask a MechEng for the details)