Slightly different situation, I know. But regularly used to book:

Bangkok-Saigon-Hanoi
Bangkok-Saigon-Taipei
Saigon-Hanoi-Taipei

I often found that Business was cheaper than Economy even with plenty of availability in both. Its seemed to be because Business allowed stopovers, so the fare was priced as one single but with a stopover. The Economy fares didn't allow this - so you had to book 2 separate singles which worked out more.

Presumably there should have been an unrestricted Y fare that also allowed stopover. The one time my agent found it, it was still more expensive that the C - perhaps because the C was discounted in some way.

Haven't tried it for a while, so perhaps it doesn't work anymore.