I had the same problem with mine when I first built it. Just speculating, but I"ll go out on a limb and say I"m probably pushung a few more ponies than you and this has worked for me. It doesnt come out of 3rd gear, the tractor just stops moving and the gear shift doesnt move, right?
make sure you replace the all the bearings on the top shaft 2,12,15 and 24. Mic the shaft and make sure it fits snug on the bearings. Also check your case and make sure it isnt wallered out where the bearings go.
Make sure the shift rail adjustment is correct, and it sits in the middle of the detent. you can also shim the detent springs and grind a little on the shaft to make a little deeper.