const max = (a, b, c) => ([a, b, c].sort().pop());

if i'm writing something SOOO performance critical that i can't afford the extra One comparison that comes with a sort i'd write it as

function max(a, b, c) {
    if (a > b) {
        return Math.max(a, c);
    }
    return Math.max(b, c);
}

because at that point you're doing too many things for it to make sense as a one-liner

this is all assuming Math.max can't take a third argument, or an array argument or something. which seems to be the assumption made in the premise