Photoshop cs6 extended serial number not accepted on a third computer

for perpetually licensed apps (eg cs6), installations aren't tracked/limited by adobe. activations are tracked by adobe.

if you purchased from or registered with adobe, you should be able to check your account and deactivate your cs6 from the unused computers.  otherwise, contact adobe support and ask for an "activation count reset".  ask for nothing else related to cs6 or they may refuse all aid.

there are 2 ways to contact adobe; chat and twitter:

chat:
use a browser that allows popups and cookies
and click here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human.

twitter:
tweet @AdobeCare

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative. ie, double check for an employee badge if contacted in the forums and look for an adobe.com domain in the email address if you click reply to an email. then check again and be very suspicious. any other method of contacting (or offering to contact you) is almost certainly a scam, https://community.adobe.com/t5/using-the-community-discussions/staying-safe-in-the-adobe-support-community/td-p/12919684/redirect_from_archived_page/true

If you didn't deactivate them when they were removed, then it will still be as if they are on that computer. You can try following the instructions from this page to deactivate the apps from your Adobe account and see if that works ( https://helpx.adobe.com/creative-cloud/kb/activation-limit-reached-creative-cloud.html#sign-out-account-page ).