It's (b). And yes you are correct, book separately then TCP.

It's possible however that after booking the first two, the inventory available for the third one could change, although it should still be there if you do it immediately. (On the other hand if you wait a while another ticket may even open up at the lower price - you never know.)

Also, do look at Skyscanner using e.g. US, France etc country setting - there may have 3 seats available through another POS at the lower price.