Originally Posted by volar

Only the person making the booking needs to be a GCH, the other passengers do not (in fact, the GCH doesn't even need to be a passenger on the booking, but would need to make the booking).
I can't see there's any limit to how many you can book - it's probably only limited by the available seats in the cabin and the number of Avios you have! Certainly 3 will be fine, assuming there are 3 seats for sale in the cabin you want to book.