It would effectively be a dead giveaway. Exhaustively searching all combinations of three characters takes trivial time even in an excruciatingly slow hash.
Hashing passwords is not done to prevent MitM, it's done to prevent/delay password disclosure if the database is hacked.
Entering partial passwords prevents MITM. OP was just explaining how partials could be implemented securely by hashing each combination; he wasn't saying hashing prevented MITM.