Note that "gonna" is very informal, and you should use "going to"... If you didn't know that, then you will have better luck searching now.
Now, about sentence 3:
You know what would probably happen?
We can shorten "Do you know" to "You know", or even just "Know". That is probably what this sentence is doing - it's asking "Do you know what would probably happen?" You're right in thinking that "would probably" indicates conjecture here; a hypothetical situation. But would doesn't place the action in the past. You would use "have" for that - Do you know what probably would have happened?
(3) is about the future, in a sense; but it isn't really about time. it is saying that in general, if the condition that is being discussed is ever met, the speaker has a good guess at what the result would be (and he is announcing that he knows, and getting your attention.) You (the listener) could ignore the speaker, and he would tell you his guess. Or you could make eye contact to show you are paying attention, or you could ask "What?" Or, you might answer with your own guess:
Speaker: Ya know what would happen if you bought candy?
Listener: I would eat all of it.
It is not talking about the past; it is talking generally - it is just talking abstractly about a cause/effect relationship.
But this thing: You know what was probably gonna happen?
This construction is about the past. It is a pretty rare construction. I'll give an example use case for it -
Bob: Charles threw his ice cream in the trash! What a waste.
Dave: You know what was probably gonna happen?
Bob: No, what?
Dave: It was probably going to drip on the carpet.
Because of the word "was", the sentence is speaking of the past. Whatever it was that was going to happen is no longer going to happen.