How large are these fans? Mobo fan headers usually don't support high output fans! ie., too large a fan, and you'll burn off your fan headers!

Generally the mobo headers only support 2-3Watt fans - probably should check your manual. Safest thing to do is to go with a 3-4pin adapter. If you want tach monitoring, you can get adapters that allow you to plug into the 4-pin PS connector and has a separate connector with the tach output to plug onto the mobo header. GL.