i have 3 yr old macbook pro 13 inch what is it worth

Check eBay & Amazon for current pricing. Also the devices condition plays a large factor in its price. If it has no defects I would estimate between $500 - $800 depending on cosmetic wear and specifications. I.E: Processor, Configured RAM, Integrated/Discrete Graphics etc.

You could try putting your computyer's specs into a site like http://www.mac2sell.net/