Contact the owner you bought it all used from and see if he/she didn't inadvertently forget to include the 3rd disk. If the original owner no longer has the missing disk then contact Garmin, explain what has happened and ask for a replacement disk. I suppose it gets worse if that fails...
I did e-mail the past owner, I thinks there is only the 2. but he looking for the 3rd. If he can't find it, Contacting Garmin will be my next step. Thanks.