I have set up the Steam startup command, and the triple image only displays on my middle screen. I am not able to see a 48:9 aspect ratio so I cannot set it.
You need to use nVidia Surround, AMD Eyefinity or custom resolution that spans all three monitors (or programs with similar functionality). Without that Raceroom cannot use three different monitors separately. Also the startup parameter is not required, most things can be enabled/set up inside the game now.
With AMD look in Configuration -> Screens -> AMD Efinity It sets an extended screen. I was also using Display Magician to switch between 1 big screen and 3 displays but does not seem to work too well latelly. Another option is some linux distribution + gamescope + luck