While there are significant differences between 2 and 3, mostly they aren't that big of a deal in terms of learning the language. There's no reason you can't learn both simultaneously, and its not hard to learn one from knowing the other, so its probably less important than you might think which you choose to learn first.

If you are going to do new greenfield development with no particular expectation of being tied to 2, 3 is probably better, But there's still some things you might wish to leverage that might make knowing 2, and how it differs from 3, useful.