You can get 3 pin extensions (not hard to find if you look). As for converting a molex connection fan to the one like on your mobo, that I'm not sure of. I've seen the opposite, where they take the 3 pin fan and allow you to connect it to a molex feed. I suppose, if you're good, you could get the plug and just convert one of those adapters.
Generally, if I want a fan to feed from the mobo, I make sure it has the proper connection on the wires. It might cost a dollar or two more, but you save that in trying to create adapters and/or time and effort...