hey111:
3 pin plug is prewired to a sensor. What plug do I use to connected the 3 pin plug to the UNO micro-controller arduino board?
Without seeing a picture I will assume you have a senor that is wired to one of those 3 pin Asian 'buckle connectors"? If not then you will have to link us a picture. There are no 3 pin mating connectors on a Uno board.
If so they are designed to plug into a 'sensor shield' also made by several Asian sellers. E-bay is full of such products from the far east. Here is a picture of such a shield
http://www.emartee.com/product/41798/Arduino-Sensor-Shield-V4.0
Lefty