Function select3 

pub fn select3<A: Future, B: Future, C: Future>(
    a: A,
    b: B,
    c: C,
) -> Select3<A, B, C> 

Poll three futures concurrently, returning whichever completes first. Poll three futures concurrently, returning whichever completes first.

Biased: polls a, then b, then c. When multiple are ready simultaneously, the earliest in order wins. The losing futures are dropped.

Useful for client + backend + timeout patterns.