We bought a $7.50 Roadmaster 15-speed mountain bike from Craigslist, but it needed a little work. Friction shifters were rusted up, so we went with MRX Comp for replacements.
Works great on the 3-ring front, and works fine with the no-name brand front derailleur on the bike. My daughter loves how easily these shifters work for her, and so far we've had no issues.
Installation was easy. Dad has never done a shifter replacement, so it was intimidating, but the included instructions were easy to follow.