You were close - "Ce qui se passe" and "Ce qui se passera" are both good, colloquial structures. You could also use "Ce qui va se passer" as an alternative.
"Ce qui s'a passé," however, is incorrect because "se passer" is a reflexive verb, which means that in the past tense it uses "être."
So you get "Ce qui s'est passé" instead.
Now, if you are asking "What happened," "What's happening," or, "What will happen?" you will need to change those phrases a little bit, using interrogative structure.
In the order you asked, you would say:
"Qu'est qui s'est passé ?"
"Qu'est-ce qui se passe ?"
"Qu'est-ce qui se passera ? / Qu'est-ce qui va se passer ?"