Re: Stations that have had a primary affiliation with all of the "Big Three"
Channel 7 in Boston (now WHDH-TV, ex-WNEV) has had all three as primary affiliations - first CBS when it signed on as WNAC-TV in 1948, then ABC when CBS went to new WHDH-TV 5 in 1961, then CBS again when WHDH-TV was replaced by WCVB in 1972, and then NBC when WBZ-TV went to CBS in 1995.
WMAR-TV 2 in Baltimore started out with CBS in 1948, swapped with WBAL-TV 11 for NBC in 1981, and then went to ABC in 1995 as part of Scripps-Howard's corporate ABC deal.
What's now KUSA-TV 9 in Denver started in 1952 as KBTV, a primary CBS affiliate. CBS went to KLZ-TV 7 (now KMGH) in 1953, leaving KBTV as DuMont/ABC and then eventually primary ABC when DuMont folded. The 1995 affiliation shuffles (sense a theme here?) turned KUSA-TV into an NBC affiliate. (Come to think of it, KBTV/KUSA may be the only station out there that's been a primary affiliate of not only each of the "big three" but of DuMont as well!)