There is an error in the first sentence. It should be:
What would happen if this park was demolished?
What would happen if this park were demolished?
The 'were' is a traditional English subjunctive that is falling out of fashion, but does not change the meaning of the sentence here.
This first pair of sentences asks about a future hypothetical event -- the future demolition of the park.
By contrast, the following question asks about a past hypothetical (or 'counterfactual' event) event -- the past demolition of the park, an event which never happened but is being considered for discussion.
What would have happened if this park had been demolished?
Another difference is that the last question is asking about past, present, or future consequences, whereas the first pair of questions is asking about present or future consequences alone.
For this reason, the last sentence can be answered with the following, but the first two could not be:
The walking path would not have been constructed.
In other words, because the park was NOT demolished, it was possible to construct a walking path (and the construction actually took place.)
If the question was instead present/future directed, it would look like this:
What would happen if this park had been demolished?
The above 'answer' would be inappropriate to this question. Instead, you could answer like this:
We would be able to build the condo.
The question "What would happen if this park had been demolished?" is an unusual form and the only difference as compared to "What would happen if this park was demolished?" is that the demolition of the park, at the time of asking the question, is presented as unlikely or impossible.