Is a 2.5 Ampere relay sufficient for 24V thermostat AC only application?
I think the answer that you want is that the 24v is only used to close relays, so the amperage needed is only for the combined amperage of the coils on the relays (or not combined if you're using one relay on the board for each control wire).

I would assume that a 2.5A relay is more than enough for each control wire, but if you are trying to drive all three units from one relay, you might start to run into issues.  For instance, if the coil you are driving takes 1/2 an amp, you're ok, but if it takes 1A, and you need to drive three of them, the Pi might start to have issues.  If you are going to combine green and yellow you are powering 6 coils, then you're probably going to be over amperage - it just depends how you plan on wiring and controlling.

The relays on the control boards of the HVAC might be marked with the coil voltage and amperage, but you might just have to measure it yourself.  Even if the amperage is too high (or just to be safe), you could use the Pi to drive a higher amp relay that in turn will switch the control wires.