If 3rd gear is gone and reverse is not functioning correctly... the problem is the direct clutch pack.
this pack functions in 3rd and reverse. The pressures are higher in reverse than 3rd, this is why you can loose 3rd and reverse still works a little.
I imagine an over haul is needed on the tranny as mentioned.
Dont worry how old you are... it is the mileage on your body that adds up.