A904 late shifting into 3rd gear

What is the 1-2 shift like?

Believe it or not engine tune has a lot to do with how the trans shifts.
I'm not saying this is what the problem is, but if you have to push the gas a lot to get the car to go then the trans thinks it should wait to shift until later.

If it shifts normal (like at 10 mph under normal cruise throttle) then you may have a valve body problem.