Pharaoh to Ring: No idea.
Dust to Skull: No idea.
Skull to Dive: Good question.
OOH! OOH! I think I think I can figure those out!
Pharaoh to Ring: Ringman being fast, I imagine he's rather light-weight, so a heavy impact, which I imagine Pharaoh Shot to be (especially when fully charged), would be a bad thing for him. I also imagine that he overheats easily, since Pharaoh Shot is a fireball, and, in his appearance in Megaman 10, he's weak to Solar Man's Solar Blaze.
Dust to Skull: Looking at Skullman's design, there are weak points in his armor, which I would assume the Dust Crusher's shrapnel hits when it explodes.
Skull to Dive: Since one of his attacks is to RAM you, he's charging head-first into a barrier. Hmm. Bug on a windshield anyone?