I'm sure
bealine is right about the return leg being cancelled - the way round this is to get two tickets, one A-B-A and the other B-A-B, the second half of each ticket being thrown away if not needed.
As an example, my security consultancy work often requires me to go to a client for a day or two to 'do my thing', come back home for a week to record my findings, and then return to client to present my findings. Hence a typical itinerary might be:
Day 1 - early LHR-FRA
Day 2 - evening return from FRA-LHR
Day 8 - early LHR-FRA
Day 8 - evening return FRA-LHR
It's
much cheaper to book two tickets, say BA Day1 LHR-FRA, day 8 FRA-LHR and LH Day2 FRA-LHR, day 8 LHR-FRA, than to do the obvious! Sometimes the client changes the date of day 8 to day 9 or 10 and, since the tickets are usually not flexible, I have to buy another, but it still works out cheaper for me (and the client, who's usually paying
). I don't know if this would work just as easily for Antalya, but I don't see why not.
Final 3 Greens, if your itinerary is A-C-B-C-A, then you need returns A-C-A and C-B-C (I suspect this is what you meant to say and the hypothetical destinations have caused confusion).
AFAIK there's no problem with this, although I'll let the experts have the final call. I don't know if A, B & C are all european destinations. If this is a long-haul trip, then there are all sorts of issues about stopovers etc that could complicate matters still further.