Cheap Multi City International FlightsLow cost international flights with several cities
Start with A->B->A and then, separated, move to either direction, where the latter is within the amount of your stay in City B. This is the major problem because they are standalone issues. In case the air fare A->B is late so that you miss B->C, then you are liable for the purchase of a new B->C-Fare.
As you are planning to stay in class 2 before going on to class 2, this will only be a minor issue on your way back where a lag in class 2 may cause you to miss class 2. Make sure you have at least 3+ hour stopovers on the way back (more if your ticket costs B->A).
After all, it is a good idea to calculate single flights from airports and airports with the following connections: C->A, C->C, air, and sea. Normally you don't normally cut down on two round trips, but sometimes it can only cost a little more and you get a easier destination C->A. Even if an air carrier has its turnstile in A and operates flights to A and C (but has no flights between A and C ), it can still offer you a plane back seat with your feet around your back (A->B and C->A).
I' ve recently made a journey like this with A Reykjavik, B Washington D.C. and C San Francisco on it. The easiest way was to use A->B and C->A (via Seattle) as round trips with one carrier and then B->C as a seperate one-way ticketing with another carrier. To do it this way was actually a little less expensive (if only a little bit), but the home plane home over Seattle was much quicker than returning home over D.C.