So, back in the day (WWII), Marvel's "Big Three" would have been ... Captain America, Namor, Human Torch? At least, I think they were the most featured on covers. They retroactively became the Invaders.
DC's would be Superman, Batman, and ... Robin?
All-American (DC's sister company - I still don't understand that relationship) would be ... Wonder Woman, Flash, Green Lantern? Hawkman?
Nedor frequently teamed-up Black Terror, Pyroman, Fighting Yank, and Doc Strange - it varied, obviously.
What about other companies? Who do you think their "Big Three" were.
I think we had this discussion before, but I can't find it.