several things can cause the converter to lock. a stock converter should not be locked under power, or lets say heavy acceleration. some aftermarket converters can be locked under full power, but those are pieces designed for that purpose.
first off, do you have a aftermarket prom or piggy back chip? some early chips locked the tcc under WOT.
second, do you have any sort of scan tool that reads data? the ecm may be sending the signal for tcc apply. a quick pass with a scan tool will tell you.
third, are you sure it locks up in third and not second gear, reason i ask is its common for the tcc solenoid to fail, causing lock up right after it shifts into second. this usually accrues after the trans gets hot-160-200f
93 is a funny year. in f body cars they still used a 700r4, everything else got a 4l60e. the 700r4 trans produced that year is very different than any other 700r4, different case, input drum, pump, valve body and many other 1 year only parts. fucked up piece IMO. you can tell which one you have by seeing if it has a TV cable attached to the throttle body. if it has a cable, its a
700r4. if so, unplug the trans, (plug located drivers side 4-6 " behind the shifter.
if it locks up with it unplugged, the solenoid is bad. they make aftermarket ones designed to splice in place, without removing the harness.
hope that helps-rich