Flag of Bohemia.
The blue triangle on hoist side was added when Czechoslovakia declared independence exactly because it was identical to the Polish flag. Slovakia didn't have a flag of their own before, I think. After dissolution, they were supposed to abandon the common symbols, but Czechs violated that because they don't have any other historical flag to use.
Moravia doesn't have its own flag either, and the unofficial one is identical to the flag of Prague (yellow and red). So they also can't just combine both flags.