Which hidden characters are you talking about?

If you're talking about Meat, Noob and Goro then Meat can be played as by finishing group mode. Noob and Goro are not in the arcade version.

If you're playing a machine with 3 "?" question mark boxes in the middle of the selection screen then that means you're playing an old Revision 1 machine. Those characters are not hidden, they are simply not in that revision. They are Cage, Jax and Reiko and were added in later revisions.

and if your refering to UMK3 (which was the 4th MK game made), then the 3 hidden characters are Ermac, Classic Sub-Zero, and Mileena. But maybe the guy above answered your question.