Union of multiple observables

Is there an operator or an easy way to generate a union of elements from different observables? Meaning that only elements that exists in all observales should be pushed downstream. Would be also great if there could be an option to define the comparator.

Meaning, I would expect:

```
A = [1, 2, 4, 5]
B = [2, 5, 7]
C = [0, 1, 2, 5, 7]

UNION (A, B, C) = [2, 5]
```
Maybe something like this exists in the RxJava itself?