I would like to setup 3 x 32inch monitors; would it be better to use Matrox triple head, in stead of connecting directly to GPU ? with 2080 GPU my FPS drop was high, now i have RTX3090, but the triplehead could reduce the FPS drop as it behaves as 1 screen ? or not ?

I cant answer your question about the triplehead, but when I ran a 3 monitor setup I saw about a 30% fps drop if I remember correctly. If you are running 3 4k monitors vram could be a concern, but I see you are running a 3090 so you should be fine. I do not believe the triple head would benefit you in anyway honestly. Those were mostly around before modern video cards had 3 monitor output.