3c is the triple compound version, best of everything idea.
MAXX SPEED
Used primarily in cross country and enduro tires, the two compounds in the outer layer are specially formulated to reduce rolling resistance and optimize treadwear and traction.
3Cmaxxterra
MAXX TERRA
An intermediate compound configuration used in select mountain tires. 3C Maxx Terra is softer and offers more traction than 3C Maxx Speed, yet provides better treadwear and less rolling resistance than Maxx Grip.
3Cmaxxgrip
MAXX GRIP
A new name for the downhill 3C compound configuration that you have come to know and trust. 3C Maxx Grip offers the ultimate in grip and slow rebound properties for unparalleled traction in downhill applications.
http://www.maxxis.com/other-bicycle-information/technology/compounds
Got one of them on and it works well.
If you want to check what you can have they have a list of every configuration
http://www.maxxis.com/tires/bicycle/mountain