The main voices are that of 3 members of the rock band, Tally Hall. Their credits are as follows:
Zubin Sedhi - "L.O." (Yellow monster)
Joe Hawley - "Bluz" (Blue monster)
Rob Cantor - "Frred" (Red monster)
Listen to the band, Tally Hall. Their catalogue consists of amazing things including many short films, videos and innovative music and have a warm, happy-go-lucky presence about them.
http://www.tallyhall.com