The easiest way is to have those on separate platforms. My main is linked to Steam, so I had to log off from Steam and create a new Steam profile. Then the 3rd on Epic. But after a much back and forward I had to go and clean it all up. Also there is the startup, if you use tools like EDMC. So here is what I did.
3 different launchers on 3 different platforms: Frontier, Steam and Epic, just buy and download/update. I cleaned up every profile to match name and email with in game commander game. Important note is that the game is not started until the client itself is showing, then close the launcher and start next. The order I have to use is:
1. Epic launcher. This has to go first. It will detect if another version with the same file name is running and then will not start if so.
2. Steam launcher, easy just press play and wait for the client.
3. Stand-alone launcher, same press play to have fun.
4. EDMC, it hooks up to the last version launched, in this case the stand-alone, my main profile.
The drawback is that if one of the instances crashes to desktop I have to close the other two and repeat the same process. That is also why I have them on different platforms. No need to log off and switch or anything like that. All are logged in with their current commander ready.
This setup is not very network demanding. I currently run on a Wifi connection to a router on another floor. The speed is 500 Mbit fiber, and pretty far from central hubs. I guess some 40-50 ms from closest server. Have not tested, but I know it will take 35 ms just to leave the country. I run all instances on max settings, even just activated 4K (using DRS). The settings for keyboard and such are located in the [appdata] and are common for all, one setting to rule them all.