Yes, anyone with a "handyman's" streak can install a seat cover set
for the TR3. I've done two of them with another one in the wings.
Just take your time, be sure and remember how the original set was
installed as you disassemble the old seats. If the plywood inserts in
the seat backs are shot, you can replace them with new by working new
plywood slowly into shape and re-riveting it with either pop rivets or
the original split rivets if they survived the disassembly. Paint the
seat pans with an anti-rust treatment and a finish coat. Use a good
brand of contact cement for the seat pan edges (I haven't tried the 3M
spray-ons, but I may next time!).
You will need a pair of "hog ring" pliers and the hog rings (hog rings
are shaped like the letter c and are used to secure the upholstery
material to the bottom of the seat springs). The tool is available
(and cheap ones work fine) from some auto parts stores and perhaps
Eastwood or TRF has them also.
Good luck, and keep the list informed of your progress.
'58 TR3 in boxes
Palo Alto, CA