Of course they all "exist." We have this unaccountable tendency to assume that the word "exist" is a property describing an object's relation to some sort of objective reality, i.e., a reality beyond our perception and understanding of reality. But any understanding that we have of reality comes through our own subjectivity, rendering access to the "objective" world inherently impossible. Even if it's there, we don't know ANYTHING about it (or more specifically, we don't know if we know anything about it. Our subjective experience of the world could be absolutely objectively correct and we'd never know, because we can't step outside our subjectivity to check). So when we say that something "exists," we can't POSSIBLY mean that it exists in an objective sense, because we don't know anything about that objective world. What we really mean (whether we know it or not) when we say something exists is that it exists in our subjective understanding of the world. So if your subjective understanding of the world includes gods from every religion, then they most certainly exist.
But just because some god exists for you, don't fool yourself into thinking that it exists for everyone else, too. And by all means don't fool yourself into thinking that just because a god exists in your subjective world, it must exist objectively. You can't examine the objective world to check!
