+1
Sounds 100% like someone has done an ATF change and decided that nice big bolt on the top of the box (3rd gear anchor bolt) looks like a great fill plug - DOH!
Yes, if the cause of the 3rd gear failure is because of the 3rd gear anchor bolt then this is recoverable and a relatively easy but slightly fiddly fix.
Do a search on here for 3rd gear anchor bolt and hopefully you'll find plenty of info on the subject along with directions on how to repair/recover 3rd gear. Of not then post back here in this thread and I'll find the posts that I know of.
