http://www.amazon.com/microSHIFT-Shi...SNYS9KEDG652BN
3x7 shifters, front derailleur, and rear derailleur for under $100.
The RD is a short cage though, which is odd since they have a long cage and its a 3x7 system. You could sell the short cage or just keep it as an extra. Pick up basically any long cage indexed Shimano derailleur from the last 35+ years and itll work.
http://www.amazon.com/Shimano-Tourne.../dp/B00GQCC9A0
This is also an option as they are 3x7.