It would not be unheard of to have a condensing unit 1/2 ton smaller than the coil. I think 2 tons smaller is out of the question.
You would have to ask Trane if the two stage unit would work on that specific coil, or with the blower set up in your furnace. You might need a different thermostat also.
A new 3 ton coil would be a minor expense compared to the condensing unit. As far as I know, Trane is not doing dry 22's, so if you are not already on an R410 unit, the cost to flush your system would be almost as much as just getting a new 3 ton coil to go with the R410 condensing unit. I am pretty sure the 2TTB is an R22 unit.