Trading prices for different platforms (Epic/Steam/GOG)

Depends on price difference/availability of a game on Steam vs the other platform. I don't think there's much of a difference anymore for Borderlands 3 cause a lot of key sites give the option between Steam vs Epic, but for something like The Witcher 3 or GTA V where the only way to get a Steam copy is the Steam store, getting a copy from GOG/Rockstar respectively would probably be much cheaper/easier. Same with most Ubisoft games and getting them from Uplay instead of Steam.