How to properly represent a tic tac toe board to a CNN?
I used a tic tac toe board to explain the value of convolutions in my master's thesis (https://apps.dtic.mil/dtic/tr/fulltext/u2/1046500.pdf checkout page 21-23 if you are interested).  I think this illustration (extracted from my thesis) might answer the question on how you could represent the board to the CNN and what you hope your filters might learn through training. Presumably you could train the network to recognize playable spaces and predict best X/O placement as you add more layers to your network.  While this doesn't tell you how to do this, I think my diagram will be useful in developing what type of input you should provide your network and some of the dimensions in order to answer your question.