Sounds like a chipped tooth on the 1st-reverse gear.
Had this happen to my first TR3, drove it for a couple of years with no further problems.
Drain the oil from the trans and if the tooth (or chip) doesn't come out with oil, fish around in the trans with a magnet. Once it comes out and you don't push 1st too hard, all will probably be ok.
OR
rebuild the trans and replace the broken gear.
There are only two options to be considered.