Keith,

Do you mean the shifter is stuck in 2nd or the transmission itself won't
shift to 3rd?  If it's the shifter, I would say the cable is probably shot.
You can replace the cable by removing the console from the interior and
getting to it from under the car.  To verify that it's the cause, remove the
lever arm from the frame to the transmission and try shifting through all
the gears with the ignition turned off and the wheels blocked.  You should
be able to turn the gear selector on the side of the transmission by hand.
If it won't budge then I would say you have a problem possibly warranting
removal of the transmission.

Most likely it's the cable.

Hey Keith,
   I had the same problem in my 1974 Camaro, long time ago. I ran over a large
tree branch or something, and the trans got stuck in first gear. The cable was
locked up, if you yank the console, it should be a relatively easy fix, and not
that expensive. If I were you, I would crawl under the car, remove the shifter
cable from the transmission lever on the side of the trans, and see if the
lever moves freely. If it does, then you should be home free with a new cable.
I've very rarely seen the stock auto shifters in those cars go bad. Good luck
Jeff