I too suspect that the linkage is not aligned properly. When it stuck in 3rd, were you trying ot downshift to 2nd, or were you going up to 4th?
As for the jumping out of 2nd, there is a fix for that, but it requires modifying the detents on the side cover. Take the shift shaft for 1st/2nd out of the cover and weld up the 2nd gear detent in it, then grind it down smooth just like there was never a detent there. Now take a die-grinder and make a new detent that lets it go further into 2nd gear. You will also have to re-shape the profile of the curve where the detents are so that it will clear and line up properly with the detent ball in the side cover. It will make more sense once you pull the cover off and look at it.. I have done that modification to about 8 of them now and it works every time. T10 is famous for that issue. The synchro teeth on the gear wear and there is not enough surface left for the slider to get a good bite on it. Usually will jump out on deceleration. They also make what are called torque-lock sliders that have back-cut grooves in them that make the slider want to pull further into gear when torque is put on it in either direction, but my experience is that by themselves, they don't always fix the problem.
