It may or may not be triple compatible. ST-4500 and 4503 will work with either double or triple. ST-4501 will work only with a double. If you can't find a model number, you can test yourself. Downshift all the way, disconnect the cable at the front derailleur, and see how many upshift sweeps you can make with the brake lever/shifter. For a triple, you need to be able to make two full sweeps, not counting any trim shifts you may feel/hear.

What are you trying to accomplish? If converting a bike from double to triple, there will be a lot of things to change over.