It doesn't sound good that only third gear has a problem
Maybe the only thing you might want to do is replace the TV cable and adjust the TV cable as "GM recommended" setting.
At one time my TV cable had a little drag when I slide the inner cable up and down, so I replaced it and made the "GM recommended" setting.
If all is replaced and adjusted and still slipping in 3rd gear, then ask one of our transmission builder board member of his opinion, since it might need to be rebuilt..