Are you supposed to know who you're playing as in Metal Gear Solid 3?

You are immediately supposed to know you are not playing as Solid Snake. The game takes place in 1964; Metal Gear takes place in 1995 and stars Solid Snake, who looks the same age as (or younger than) Snake in MGS3.

If you have played MGS1 / MGS2, you know Solid Snake is a clone of Big Boss. Well, here's a game where you play a character who looks exactly like Solid Snake, 30 years earlier, and has the codename "Snake". Who could that be?

As someone paying attention to gaming news when MGS3 was being previewed, Kojima was coy during interviews, but "everyone" knew that this game starred Big Boss before it came out.