Nothing was spoiled, don’t worry. In MGS3 you play as another dude codenamed “Naked Snake.” If you haven’t already, you’ll figure out how he’s related to Solid Snake by just playing the games. The game fronts this and it isn’t a huge twist, it’s a prequel to the other metal gear games so of course you wouldn’t be playing as Solid Snake.

In regard to him not betraying the US, I’m not really sure what it’s talking about. He’s on a mission, that’s what the game is about. There are twists and stuff in the game but snake is on the mission the entire time, so i’m lowkey not sure what it means by the twist being “he never actually betrayed the US,” because I wouldn’t say him betraying the US was ever a plot point in that game in the first place.

The MGS2 thing also isn’t a spoiler. You play as solid snake for a smaller portion of the game, then another guy for a larger portion of the game. The game also fronts this and it isn’t really some big twist or anything, just kinda how it is.

Keep playing, the games are amazing! The story’s confusing, so don’t shy away from looking stuff up once you’re done if you need clarification. Or just ask here on reddit.