The 3DES license gets carried over when you update your PIX code. To check
if you have a pix that is 3DES enabled, do a show ver, and it would tell you whether 3DES is enabled or not. If you don't have it enabled and you don't have the key, you could either email licensing@cisco.com, or log a TAC case
stating your PIX serial no., proof of purchase of 3DES keys (most of there is some paperwork included with the PIX about 3DES licensing if you have purchased it), and ask to provide you the appropriate key.