If I understand correctly, you have 3 pin fans you want to act as four pin types and want to know about about a possible adapter for that. It doesn't exist but I will link you to this thread from Overclockers that shows the schematics for making your own http://www.overclockers.com/forums/showthread.php?t=683...
Or, you could get a fan controller (rheostat) and use that to control your fan speeds. I think that's probably the easiest route.
The fan header connectors are backwards/forwards compatible with each other, thats why they have guide grooves to fit the guide tab on the mobo fan connector, the only difference with the 4-pin connectors is they support PWM speed management.
If you have 4 pin fans you want to install and have (for example) the Coolermaster Cosmos II case with a fan controller and only the 3 pin power cables from the controller, then these adapters are very useful.