I've never seen any adapters around for this but I could be wrong.
I'm assuming that the 3 pin connector has just 2 wires, pin 1 & pin 3, with the middle pin having no wire.
If so, there are 2 things you could do :
1. Slice the connector in half down the middle, leaving 2 single pin connectors (may need to file the cut edges back a bit).
2. Find a spare used 2 pin connector from an old machine or whatever, with 2 wires in. Chop off the 3 pin connector & connect the wires from the 2 pin straight to the remaining cable. (obviously make sure that the connection is well insulated)
Alternatively, if you have a spare led from an old case with a 2 pin connector on, you may be able to swap the entire thing over.
I know these are messy solutions & not ideal but I've done it before a few times without a problem.