I have a dual head configuration with an Nvidia graphic card, which
perfectly works. but the monitors are standard ones (LG, resolution:
1280x1024) and I would like to add a monitor for treating my photos
Have you considered replacing the two small monitors with a bigger,
higher resolution one? In some markets, the cost of a WQHD (2560x1440)
or UHD (3840x2160) IPS or AHVA panel has come down a lot recently.
You don't mention what kind of panels you have or want, but as I'm sure
you know a good panel type is important for colour accuracy.
Another detail you don't mention is which driver you're using: the
vendor-supplied nvidia driver or nouveau. For the nvidia driver, try the
"nvidia-settings" application, as it provides much more control over
your displays and screens.
I am wondering if I could add another graphic card in order to have
a third monitor with an HD resolution (1920x1080 or more) and a
specific color profile.
Yes, you can add another video card if you have a free PCI port on your
motherboard, but for the cost of a second video card and third monitor
(in both money, space and convenience) there may be better choices.
With a single good quality, high resolution monitor connected to an
adequate video card you can use an almost unlimited number of workspaces
to effectively have the whole screen available for as many applications
as you want.
The colour profiles are managed for each device. Look in "Color" in your
Settings and you should see each device capable of supporting a profile.
You will probably see that a profile has been automatically loaded.
-Andrew J. Caines- Unix Systems Architect A.J.Caines(a)halplant.com
"Machines take me by surprise with great frequency" - Alan Turing