How is (1-3m)%3 is 1

I didn’t understand that how is (1-3m)%3 is 1.
At 14:36 timestamp in the video he said (1-3m)%3 = (1-0).
In the beginning of the video he said (a-b)%m = (a-b+m)%m
according to this how is (1-3m)%3 is 1