Sign out the other two computers and you can sign in to the 3rd one. You can do that on adobe.com or from within your application or from within the CC application.
Having an all apps plan includes the photography plan and makes that one obsolete. There is no real reason to have that in addition to the first one, except if you have concurrent use for Photoshop or Lightroom. There, however, I guess you would need to have 2 different Adobe ids.
Anyway, you will need to Contact Customer Care to either cancel the Photography plan or to get that allocated to de different id (basically this will be done by cancel/take a new subscription).
Personally, I have one subscription at work (payed by my boss) and one private that I use for my own work. That means that I really can have 4 active installs and 2 concurrent sessions. But I have 2 id‘s (a work id and a private one). The only reason for me to do that (besides being an ACP) is that my work license can be reclaimed any time by my boss for being alloced to a different person. (we had once that situation as we had a student doing student work and needing a license for her work).