Hi John,
Are you sure you really want to pull something with a 3? -- That said, I did. I think you'll probably have to create the hitch yourself, as it's not real common car to tow with.
I've had my TR3 since high school and back then I had a motorcycle too, so believe it or not, I actually towed my bike from Oregon to Alabama with my TR3. I had to build the hitch myself (actually my body shop buddy did). He welded a T with the two sides of the top to the frame just behind the rear axle and then put a bolt through the hole in the aft most tubular crossmember and the stem of the T. (We used U channel, the top of the T was welded at the top of the U to the frame. The stem of the T, the U channel was turned over. I think that might have been to make it easier to attach it flat to the crossmember.) I would think any trailer or welding shop could do something similar pretty easily. You could probably do it as a bolt on if you wanted too.
Good luck
Bryan