Outsiders: So back to my question of where else can you purchase games if not for those three gaming sites?
Well as many have noted, Itch.io is a good option, though it does have some pitfalls, with the biggest one for me being too much crap to wade through. But I guess you could say that last issue is the same at Steam.
Epic is not the only major game store outside of GOG and Steam, as others have also noted.
The Humble Store for the most part is just a frontend for others, especially Steam, but it does have some good DRM-Free games now and then.
I tend to categorize stores as either DRM-Free or DRM, so with that in mind GOG is the clear winner in its DRM-Free category.
As for the others, take your pick. Epic has my first vote though, as it has given the Gaming World many great free games (some AAA), in their war with Steam.
Many of the good games at Itch.io are also available at Steam and even at GOG etc.
At times it may be worth dealing directly with the Dev/Publisher of a specific game, even if you do end up with a Steam key or an Epic code etc.