It's a resistor sensitive to light. (Φωτοαντίσταση)
But they have 2 legs, not 3. Are you sure it has 3 legs???
If you know exactly the resistance range, you can use any photoresistor you find.
Like this http://cgi.ebay.com/5x-Photo-Light-D...item3ca8be132c
Some have infinite resistance when they get 100% light, some others the opposite...