[UncleBob]

uh, I wouldn't recommend anything yet. You haven't diagnosed it yet, you have a couple guesses.

The light flashing is informing you of two things, one of them you already knew, which is, the computer noticed a major issue and its in limp mode, which is why its stuck in 3rd gear. Although I'm not familiar with your particular model, I can promise you there will be some data to be had from the computer.

If you are only getting guesses from your shop, its time to take it to a tranny shop before you waste several hundreds of dollars on wild guesses. As usual, it'll be cheaper to have it diagnosed from a shop that has the equipment, experience and data necessary, than playing the "lottery" with another shop.

Very well could be TPS and/or alternator, but the shop better be sure, not guessing.