transmission problem?

I have a 1960 corvair sedan with a 3 speed standard on the floor, and I have been driving it around lately on occasional warm days, being a new owner I am looking forward to driving it around more and more. But today I went for a drive -and somewhere along the way I lost 3rd gear.. the shifter feels more loose than normal and I can successfully go from R-1-2 and then it acts like im throwing it into 1st or 2nd gear instead of the Third. Anyone one with an idea fix?

..stuck at home on a warm February day