Limited to only single expression functions, maybe this wouldn't be too confusing. That said, I like the last alternative best, but with a further limitation of just local functions only (does limiting the scope help with type inference?) and with a fat arrow. That would give programmers a real one-liner:
func multiplyByTwoAndThree(a: Int) -> (Int, Int) {
func multiply(b: Int) => a * b
return (multiply(b: 2), multiply(b: 3))
}