MMC3 has 6 switchable CHR banks. 4 of them are 1k (64 tiles) in size, 2 of them are 2k (128 tiles) in size. It can pick which side (left or right) gets the 2k banks, and which side gets the 1k banks.
Set a data breakpoint at $8000 and $8001
Watch for the command number you're interested in at $8000, probably command 0-5.
Watch for the bank number the game uses at $8001 after you hit the $8000 write you're interested in
Look at the game code, see if it just some something simple like LDA #$12 \ STA $8001.