Superman and Batman have been appearing in comics together since the Silver Age, and Wonder Woman joining in isn't rare, either. Generally they team up, though having them fight each other is always a good way to draw in readers.
Superman: "We're surrounded, you know. I can hear them all."
Batman: "I think we can take them. Do you think we can take them?"
Superman: "You always think we can take them."
Batman: "Yes, I do."
Superman: "Then, let's do it."
- If that counts, we may as well mention All Star Comics #3, the comic which created The DCU as a shared universe.