the 3 is the filter size (3x3). The input is just the state of the tic tac toe board, which is a one dimensional array of length 9.
One other thing that I noticed:
When using the fully connected feed forward, the performance is incredibly unstable when changing the number of neurons. For example when using 20, 32 and 20 neurons the random player wins about 25% after 10.000 epochs. If I change it to 20, 30, 20 it is more like 38%. Something feels really strange here.