my tranny has something wrong with the 3rd gear sincro and it would be easier to just buy a new trans

I happen to have 2 S54 transmissions sitting around... one was my old one with the same issues that you're experiencing (3rd gear syncro) and the other I purchased with the plan of swapping it out (I was told no grinding from the seller) - check out the link in my sig for pics and make an offer.