Sounds like your 3rd gear synchro is stuffed.
You can pull the box apart and replace it (though I've read only some of the required parts are still available) but in my opinion it'd be cheaper and easier to find a good replacement and throw that in. I can't comment on different oils and the magic they work.
Or simply live with it and double clutch on 4-3 shifts. It'll become second nature in no time.