Big Three

In the early days, with fandom's focus on the prozines, this meant the top three prozines of the time. (See the discussion from Fancy 1 and Fancy 2, below.)

Later, the Big Three was more likely to refer to towering figures among pros duringt he Golden Age. Two were indisputable: Robert A. Heinlein and A. E. Van Vogt who almost defined the new SF of 1938-1948. (The third of the Big Three was usually whoever among Asimov, Bradbury and Clarke the speaker liked best.)

from Fancyclopedia 2 ca. 1959
The most important stfsy pros. Amazing Stories, Astounding Stories and Wonder Stories up to the early 40s; so called because for years they were the only prozines there were except for short-lived things like Flash Gordon, Miracle Science and Fantasy Stories, and Fanciful Tales. (Not counting Weird Tales, which wasn't science fiction.) From that time to about 1944 Astounding, FFM, and Unknown; after that till 1950, Astounding, FFM/FN, and the Standard Twins; thereafter and until the present Astounding, Galaxy and the Magazine of Fantasy and Science-Fiction.
from Fancyclopedia 1 ca. 1944
Amazing Stories, Wonder Stories, and Astounding Stories. So called because for years, in the early 1930's they were the only prozines there were except short-lived efforts such as Flash Gordon, Miracle Science and Fantasy Stories, and Fanciful Tales (and of course Weird Tales, which isn't science-fiction).