By the time FANTASTIC FOUR ANNUAL #3 made it to newsstands in 1965, Marvel Comics had already become famous in comic-book circles for the art of the crossover. Crossovers at their major competitor DC Comics were more limited in nature, largely because DC was a larger company and possessed several editors for their line, each of whom tended to promote crossovers only for books they regularly edited. For the decade of the 1960s Stan Lee edited all titles and wrote many of them, so there was no resistance to having any character meet any other character-- or, in this case, having nearly every Marvel character make an appearance. Contrary to the cover, the story did not include the Sub-Mariner or the Hulk, even though they're referred to in indirect ways. The tale certainly doesn't feature Kid Colt, a western hero of the 1800s who would have been damn old if he'd lived until 1965.
The lead story, "Bedlam at the Baxter Building," is more impressive as a landmark than as a narrative. In the regular FANTASTIC FOUR title, Reed "Mister Fantastic" Richards and Sue "Invisible Girl" Storm, two members of the quartet, had become engaged, and FF ANNUAL #3 devoted 23 pages to the big wedding. The supergroup's nemesis Doctor Doom decided to rain on the wedding parade, by using a hypnotic device to compel dozens of super-villains to attack the proceedings. Fortunately, the FF has invited dozens of super-heroes to the event as well. Almost the entire story is given off to non-stop, almost incoherent action. In those days even Marvel heroes were expected to maintain a coterie of super-villains that they usually did not share with other heroes, so FF ANNUAL offered the then-rare pleasure of seeing heroes encounter villains who didn't usually hang around their neighborhoods, like Captain America fighting Thor's villain, the Cobra. The big donnybrook unleashes so much havoc that the Watcher shows up to watch it all. The cerebral alien generously gives Reed Richards a golden ticket to find a super-device that time-reverses the whole attack and makes it possible for the wedding to be concluded in peace.
Fun as this wild tale is, its best moment appears at the end, when authors Stan Lee and Jack Kirby try to crash the wedding party and are given the gate by Nick Fury.
This is my first example of a STATIC CROSSOVER, for the emphasis here is not on the particular ways that two or more characters impact on one another, but upon the breadth and diversity of the universe in which they all co-exist.