I recently drained my GT6 Mk 3 cooling system via the block drain plug and bottom hose. After replacing the radiator (which now has 50% more cores) it took over 6.5 litres to refill, so most of the coolant will come out!
The handbook recommendation for filling to avoid air locks worked well for me:
1. Fill the system via the radiator filler until full. Fit rad cap and half fill the expansion bottle.
2. Run engine at fast idle for 30 seconds, stop engine and top up system via the radiator filler. Refit cap.
3. Run the engine up to normal operating temperature. Stop and allow to cool, then top up expansion bottle until half full.
Cheers,
Gully