Do you have any lights on, check engine or Overdrive light flashing?
Is this your only concern harsh shift into 3rd?
How does the transmission fluid look?
Vacuum leaks can cause O2 sensor codes.
Here is a list of possible causes for harsh 3rd shift. The most likely of which would be the direct clutch accumulator valve, 2-3 shift valve springs. There are many possibilities that can cause this concern.