Firstly, if you haven't already, you might like to read this answer which covers some of the basics around how airline fares/rules work.
What you're seeing here is technically a round trip - presuming that you are not actually taking a stopover (ie, a stop longer than the airline considers to be a connection - often 4 hours but it can be longer) at point B on the way back.
It is the breaks in the trip that define the type of trip, not the "point of combination" of the fares. For example, if you were to fly X-Y, Y-Z and Z-X as three separate legs with a day between flights, that would be a circle-trip, and the fares used to construct that trip would need to allow for a circle trip (or at least, not have anything disallowing it).
However if you were to fly X-Y, Y-Z-X where the stop in Z was only a connection, that would be a round-trip - even if the same fares were used to construct the trip as in the first example. The difference between these two trips is not the route you took or the fares that were used - what makes one a round-trip and the other a circle trip is the locations where you actually stopped travelling and stayed in the location (Y and Z in the first example, only Y in the second)
In your example the first two fares (covering A-B-C and C-B) are actually forming an "Open Jaw" trip, as you are ending the fare component somewhere other than where you started. The third fare component (B-A) may well be a one-way fare, but the fare rules on that fare must also allow for it being used as a part of a round trip, as that's what you're doing here. (It's uncommon for one-way fares not to allow being used in a round trip ticket, but it does happen as was reported here)