You have not specified the air carrier, so this is simply a generalization. But, generally, all tickets in a PNR must be issued in the same fare basis. Thus, if there are only 2 "cheap" F tickets and you try to purchase 3, all 3 will price at the more expensive fare bucket.

There is rarely a reason to purchase tickets in the same PNR without status and especially flying in F. I would purchase each of the tickets separately. That way, the first 2 will be at the "cheap" F and you will pay the higher fare bucket only for the third ticket.

If you provide the air carrier, this can be confirmed.