The first two cases show function expressions, and can appear anywhere an expression like (1+1 or x*f(4)) would appear. Just like how 1+1, evaluates to 2, these expressions evaluate to a corresponding function.
The third case is a function declation statement, and can appear anywhere you can have other statements (like an if or while statement).
There is not much point in trying to declare an anonymous function via a Funcion declaration statements, since otherwise noone would get a reference to the function afterwards.
The reason you need the opening ( or the var x = like in the first two cases is that they force next bit to be parsed in an expression context. (just think how you cant do var x = if ..., for example). If you just put the function as the first thing it will be parsed as the declaration statement you don’t want.