if it hasn't been updated then 6k hours is about the normal life of a transmission. Not saying that is acceptable, just being honest on the basis of experience with M Series graders.
Clutch #3 is the reverse direction clutch ............. this sounds ominous.
A Serial Number is always good.
Go into the transmission suction screen first and look for a load of partciles of burnt friction material. In case you're not aware there are 2 screens, one for the suction from the diff housing, the other for the transmission, make sure you get the right one otherwise it cpould send you on a wild goose chase.
If there is nothing in the screen then post the Diagnostic Code here and I'll see if there is a trouleshooting procedure for it. You coud be in luck and find it's no more than a f**ked solenoid valve for #3 clutch.