(A || B) && C

AorBandC.swift

import RxSwift

let A = Observable.just(())
let B = Observable.just(())
let C = Observable.just(())

let AorB = Observable.combineLatest(A, B) { (a, b) in
    return ()
}

let AorBandC = Observable.zip(AorB, C) { (ab, c) in
    return ()
}

AorBandC.subscribe {
    print("we livin")
}