A triangle pattern will only make sense if the two destinations are reasonably close together and both can easily be served non stop (although prevailing winds will also need to be considered). Obviously the location with the longer runway can be used for the second stopover.
It might be necessary to have a longer stop at the end location in order to fit in with airport curfews, slot availability, etc.
It could also depend upon loads at the two stations and wish to fly non stop in both directions from the more important destination, particularly if nearer or if a rival carrier flies non stop.
The carrier may have fifth freedom rights between the two outstations which could be lucrative.