Can I add a third computer to the plan I have and how much does it cost?

As per EULA, you can install and activate it on two machines, however, you cannot use both of them at the same time. If you don't use it often you can try to sign out from any of the previously activated machine and sign in on 3rd one, else you would need to purchase. For pricing check: Creative Cloud pricing and membership plans | Adobe Creative Cloud

Check point 1.1 : http://wwwimages.adobe.com/content/dam/Adobe/en/legal/servicetou/Software_Terms-en_US-20150407_2200.pdf

Hope this helps.