Some 48:9 triple monitor [5760x1080] wallpapers I've found work well. Seems hard to find ones that don't get grainy and are also good for gaming on middle monitor only (so not distracting).
6x Space S 4K wallpaper
Star Wars 5760X1080 Wallpapers on WallpaperDog
Triple monitor multi screen multiple space earth terre wallpaper | 5760x1080 | 534094 | WallpaperUP
5760 X 1080 4K Wallpapers - Top Free 5760 X 1080 4K Backgrounds - WallpaperAccess
Free download Deep Space Moon Ultra HD Desktop Background Wallpaper for [ 5760x1080] for your Desktop, Mobile & Tablet | Explore 24+ 5760X1080 White Wallpapers | Wallpaper HD 5760X1080, 5760x1080 Gaming Wallpaper, Free 5760x1080 Wallpaper