Some motherboard have a 3 pin and a 2 pin adapters, but also a lot of cases come with the Power LED split already so you can plug it in how you want it. But yes if I use to swap out like a 4 port plastic piece for internal speaker from a 2 pin Power LED to make it fit and work on a 3 Pin Power LED.
I would go see if a local computer store has a cable for you.
It is normally the other way, you have a 3 pin and need a 2. in those cases you can just move the wire over without much problem.
If you happen to have an old cd drive audio cable, you can use that to "make" a 3 pin cable.
If you look at the plugs you will see a small tab you can lift and remove the plug and move it to whatever location you want. So if you have ANY old case or audio cable from a cd/dvd drive you can steal the end cut it down to 3 position and make a new power led plug on the existing cable.
Another image to show what it looks like when you remove the plugs so you can place them in a new connector.