Alright, I've been looking for the third nut to remove the heater case for about 30 min and I must be blind. I've removed one near the top of the firewall toward the passenger side in relation to the heater hoses and one directly below said hoses. Any one know where it is?

I believe that one is located under the A/C box behind the right side fender well on the firewall.

Use a mirror and flash light to check.