Anyone who has any experience with mtx75s should be well aware of 3rd gear syncro woes. Obviously, since they are a super-common problem, I wouldn't let others make you feel like you caused it. 3rd gear especially has always been the mtx75s problem (among other things like chatter of the t.o. bearing and not liking cold weather.) Noone's insurance, including your own, will pay for internal tranny issues, as they are no dummies and won't let you attribute that to any wreck you have been in. Your options are: skip 3rd gear when driving (like I did for 2 years), buy a used tranny that has low miles, or bravely open up the tranny and replace the syncros.
I will agree with sailor that you should probably flush the tranny a couple of times. Not only would that expel grinded bits, but it could help 'free up' the syncros a bit.