Does Rust 2018 support "if let" chaining?

RFC #2497 has not been implemented yet. The GitHub issue you linked is only for describing how to deal with the ambiguity.

To enable the second interpretation in the previous section a warning must be emitted in Rust 2015 informing the user that [...] will both become hard errors, in the first version of Rust where the 2018 edition is stable, without the let_chains features having been stabilized.

So no, you cannot use the syntax yet, but instead use tuples as a workaround, as you already did.

if v.len() >= 3 {
    if let (Token::Type1(value1), Token::Type2(value2), Token::Type3(value3)) =
        (&v[0], &v[1], &v[2])
    {
        return Parsed123(value1, value2, value3);
    }
}

While hellow is correct that RFC #2497 is not yet supported in 2018 (and 2015), I felt the if_chain library mentioned by Michail was worthy of an answer.

The if_chain library provides a macro that transforms some code that is almost in the form of RFC #2497 into valid Rust.

You can write:

if_chain! {
    if v.len() >= 3;
    if let Token::Type1(value1) = &v[0]; 
    if let Token::Type2(value2) = &v[1]; 
    if let Token::Type3(value3) = &v[2];
    then {
        return Parsed123(value1, value2, value3);
    }
}

which the compiler treats as:

if v.len() >= 3 {
    if let Token::Type1(value1) = &v[0] {
        if let Token::Type2(value2) = &v[1] {
            if let Token::Type(value3) = &v[2] {
                return Parsed123(value1, value2, value3);
            }
        }
    }
}