There is no reason TF3 board outputs can't be used on A0 or any other analog input. I have hooked up analog inputs indiscriminately. Analog, is analog, is analog. As long as the wiring map has it as an available analog input, it is available to be used, no matter what the analog input actually is coming from as long as it matches the boards' analog input specifications (TF3 board #1, TF3 board #2, current sensor, etc.).