Well I see that two ways,
First if your car isnt knocking in first and second, but a little on third without alky, why not dial it down. In other words bring down the gain/ramp. And you wont even know its spraying. Yet be effective for your third gear concern.
See its better to have a system working in case of an accident(WG,vacuum hose,etc) than not. And it wont affect your performance.
If you want to only have it trip at third gear, wire a relay across the third gear switch on the transmission, and have the relay then apply MAP signal to the PAC when third gear hits. In other words, my understanding is the pressure switches in the transmissions apply ground when activated, thats how the ecm knows the car is in third. So have a relay with power on 85, and 86 connected to the wire for 3rd. then green wire on the MAP going to 30 on the relay and 87 is the PAC controller. When the car hits third, the relay clicks and applies the boost reference to the green wire.
Another way would be using the purple(violet) wire connected to the relay. Whereby the purple wire has ground applied until third gear then the system tracks normally. And this way you would get like 15-20 PSI of alky until that wire lost ground at which time it would resume normal operation when it hits third.
Lots of options.. your call
Julio
