Depends on the game you play and the settings you select. Graphically demanding games like Metro or Mankind can be very taxing for 3 x 1440p/240hz, specially with Ray Tracing on. You will need a lot of VRAM for such games. 3090 has plenty of that, so it should be able to handle it. I think 3080 should be able to handle it too, not so sure though.
hard to answer without knowing what you are going to play with it... so a quick answer if you play CSgo then no it will all work fine but if you gonna play newcoming Farcry 6 with raytracing and 4k resolutions then yes its gonna have a FPS issue. your CPU can run 2 or even more 3090s with no problem, so dont worry that much there. tip: as unknown why you want to run 3 monitors i tried that and geniualy dont like it i get less preformance and a neck strain trying to keep up with all 3 screens and borders (betweene monitors) just kill my feel, so i made 2 be my sidemonitors 1080p60 and took a 34" 2k/144hz for main screen and i love every single seconds of it. game on my main monitor while other 2 are discord/yt/webbrowsing/workcharts. and if you just wanted that to have a huge desktop then go ultrawide monitors.