First, the dumb questions...  fluid level good?  How long ago was the last
fluid/filter change?

If the fluid is full, I'd suspect the rear clutch is slipping...  the higher
you go in gear, the worse it gets (less mechanical advantage means more load
on the drive side - same reason a worn clutch in a standard manifests itself
in the higher gears first).

Slippage in 3rd indicates it's not as simple as a band adjustment (front
band only used in 2nd).  It could be a seal that's causing a drop in
hydraulic pressure, which could be the root cause of the clutch slippage (as
opposed to just worn-out clutches - which is also a possibility).  This can
be checked with a hydraulic test (requires hooking up pressure gauges to the
various test ports, running the vehicle on jackstands, and measuring the
pressure), but the reality of it is, you're most likely looking at a
replacement/rebuild.