P3 air on the dash-8 comes off of the high pressure centrifigual compressor. It is used as bleed to the pressurization/airconditioning system, and the de-icing system, as well as to help the HMU provide fuel schedueling (-100 series anyway.) P3 air always flows to the HMU, and is turned on or off as a function of engine speed to the pressurization system by the PSRV valves. P3 air is used in pressurization and airconditioning at low power settings, and is shut off at high settings to prevent overheat of the ACM and the ducting. At high settings, P2.5 air is used. It comes off of the low pressure compressor. In flight, when you pull the power back, you can really tell when the P3 air kicks in, as the a/c system will pump out a noticable increase in air flow (to the flight crew at least.)
Good luck with the ground school. I flew them for only 6 months (now on the e120
) and I really loved them. While they weren't very fast or very sexy, they are big, comfortable, a joy to fly, easy to fly (though that perfect landing is very elusive) and great performers. (How does a Vref of only 84 kts at low light weight grab you!) I miss them a lot. Have fun.
m.v.