Make sure you do a simple test to check the motor mounts to see if that is the reason it is jumping out of 3rd. With a "3 on the tree" the linkage goes from the cab or base of the truck to the transmission, the cab is stable and solid, the engine and trans move slightly in the mounts. Block the wheels, put on the E-brake. Have some one watch the engine for you, or watch between the hood and cab opening. Put it in gear with your foot on the brake and let the clutch out slowly. You will see if the motor "jumps' or moves off the broken mount to the side or it will look like it is twisting under the hood showing a broken motor mount, also to see the other side put the truck in reverse and repeat. You are putting the torque on the engine when you have the brake on so the engine will move if the motor mount is broken. Cruising and letting your foot off the gas at highway speeds will transfer the torque to the motor temporally and may cause the trans to slip out of gear if there is enough movement… just a thought to check and do the cheapest stuff first.
