What kind of shifter? Is it had to move the shifter to the high gear position? If so:
-check cable friction, and routing beneath bb shell
Can the shifter move easily to the 3rd gear position, but the derailleur won't follow?
-Tighten the cable with a 4th hand tool
The other thing I'd suggest would be BB length, but if you got everything from a build kit, the spindle should be the proper size.