Honestly, with the amount of different payment methods throughout different countries, a billing system at the same capacity as WHMCS should have the ability to add your own payment gateways that would just basically replicate the Bank Transfer and Mail In Payment frameworks with the ability to make your own name for it. Just in Canada alone it would be expected that I should have the ability to create separate ones and call them cash, cheque, ETF, wire transfer, Interac e-Transfer.
Three is certainly not enough. How come the devs haven't worked on something like this?