Unfortunately it can’t be done. You can book for 2 or 4 people but not 3.

I don’t think BA intended this outcome and no doubt it’s simply an IT limitation. But in the priority list of IT issues to fix I suspect this is near the bottom.

The only workaround is for another of the travellers to have a voucher in their own name and you make 2 bookings, one for 2 people and one for 1 with 50% discount