997.. I need to add concentrated coolant (up to 3 gallons) to an already
filled cooling system. Which hose, under the car, do I need to remove to
incrementally drain the fluid while I add concentrated coolant into the
reservoir ? I am hoping by doing it this way, I can avoid incorporating
air into the cooling system.

 

That's tough, there is a 6mm allen drain on the oil pump housing to drain the coolant on the rear of the engine (towards the back bumper). If you do get air in the system, just have a buddy hold the RPMs at 4k while you fill the coolant reservoir. Good luck!

 

If you open up that 6 mm allen drain you will get air in...
IMO just leave it alone

 

Don't understand if the system is already filled why the need to add 3 gallons of concentrate?