I'll let someone else describe and post images to the Wonder Woman issue which predates these.
Yet the one DC built it's burgeoning multiverse off of, was
Some history:
1961 -Established and explained by Barry Allen as "two similar Universes" to Jay Garrick in "Flash of Two Worlds" The Flash #123, where Barry Allen is transported to the Earth where the original Flash, Jay Garrick, exists, and calls him back into action.
Establishing the ongoing adventures of what is now known as Earth 2
1963 -This was reestablished in the first team crossover between the JLA a JSA, in "Crisis on Earth-One" (Justice League of America #21) and "Crisis on Earth-Two" (Justice League of America #22).
Which started the yearly JLA and the JSA tradition of crossover between Earth 1 and Earth 2.
1964 - Justice League of AmericaJ #29 - Earth-Three is introduced and the mirror universe, whose history is depicted as a mirror image to that of the Earth we know. And Universe of the Earth-Three analogue to the Justice League, the Crime Syndicate of America
Described as an "Interdenominational Struggle"
Establishing Three Earths and a multiverse, although as indicated it is not officially called that.
(significant this "Mirror Earth" also predates Star Trek's famous Mirror Universe.
continued....