heater box removal

ok, i have to change the heater core. i thought while doing so id check the ac unit in the same box. ive seen where people say the entire box comes out with 3 bolts. but i can only see 3 bolts like the one pictured. i know about the one in the vent box, but where are the others. one behind the radio like in the vent box ok,, but what takes the box out of the dash so i can get to everthing?? im in need of some help so i dont' take the wrong things apart etc. can you be more specific. tks mike