I just wanted to what John had mentioned
When you install it on the third computer or for the third time & when you to activate it, if it installed & activated already twice, there will be an option of deactivating it from the previous computers, please click on that.(To deactivate from preinstalled machine, Open any Adobe CC application > Help Menu > SSign Out)
Next it will allow you to activate on your new machine,
Now you can go back to your first computer, log out & log back in of Desktopp CC App, launch any product, if there is any product to buy, in that prompt there will be an option to " license the software", please click on that & sign in.
It will reactivate the CC on the first machine.
You have already deactivated it from one machine, hence you can activate it on third computer.