var obs1 = Rx.Observable.fromEvent(input1, 'input').map((e) => e.target.value);
var obs2 = Rx.Observable.fromEvent(input2, 'input').map((e) => e.target.value);
var obs3 = Rx.Observable.fromEvent(input3, 'input').map((e) => e.target.value);

var counter = 0;

Rx.Observable.combineLatest(obs1, obs2, obs3)
        .subscribe(([ res1, res2, res3 ]) => {
  span.textContent = 'Result: ' + ++counter + ' -> ' + res1 +
                    ' / ' +
                    res2 +
                    ' / ' +
                    res3;
        });

combineLatest(
  obs1,
  obs2.startWith(undefined),
  obs3.startWith(undefined)
).subscribe(([res1, res2, res3]) => { ... })