Damn, Sorry about that. Your right.
According to this website, it shows a video how to set up PLP in Eyefinity: http://www.wsgf.org/article/amd-eyefinity-plp-portrait-landscape-portrait
It is 4 years old but should still be valid.
You can check the Eyefinity guide yourself from here: https://www.amd.com/en/support/kb/faq/dh-014
EDIT: to answer you question, I believe that it is the software that you use that determines whether PLP is possible. As long as the GPU card supports 3 or more monitors.
Otherwise I am out of suggestions.