How much energy is yielded by "fusion" secondaries ([1st]fission - [2nd]fusion-fission) varies by design, but secondary's fusion is used for more efficient (eg more complete) fission. Only huge bombs have tertiary fusion stages that contribute much more yield than fission stages.
I'm not sure what you mean, the W39 was a Teller-Ulam design bomb. It wasn't a boosted fission bomb (though maybe the primary was? I don't know). I believe the Castle bombs were as well.
According to Wikipedia, the Mark 15 had a real thermonuclear second stage that generated significant yield. However, its second stage was also packed with enriched uranium, and it obtained most of its energy from fission.
Essentially, it was a two-stage thermonuclear weapon that had a lower fusion fraction than most two-stage weaopns.
Summary of atomic weapons:
1. Pure fission. Fusion contributes 0% of yield.
2. Boosted fission: Fusion is used solely to improve the explosive yield of the fission. Fusion contributes 1% of yield.
3. Dirty two-stage thermonuclear: Fusion contributes between 1 and 50% of yield. Most of the yield comes from fissioning of the U-238 tamper by fast neutrons.
4. Clean two-stage thermonuclear: Fusion contributes 90-99% of yield. The U-238 tamper is replaced with beryllium or lead, which does not undergo fission when hit with fast neutrons.
5. Three-stage thermonuclear: Same as two-stage. Thermonuclear bombs are considered to be an "unlimited" design, because you can keep stacking stages on top of each other, until you've got enough to blow up the planet.
http://en.wikipedia.org/wiki/Teller-Ulam_design