Answer: I have no life and I’m a partial invalid.
The Defenders were created by Roy Thomas, but developed organically through the cancellation of Dr. Strange’s monthly series. He needed to resolve a storyline, and ran out of issues to do it, so he brought the good Doctor, Sub Mariner, and Hulk together in the last issue of Dr. Strange and regular issues of the other characters’ series. I guess it was helpful to be writing all three titles…
Dr. Strange teams with Sub-Mariner then the Hulk to protect the Earth from invasion by Lovecraftian interplanar beings known as the Undying Ones and their leader, the Nameless One.
So, these are the big three, allied in one adventure. But they aren’t a team. Yet.