The OED has a 1955 cite from Frederik Pohl for "tri-di". Malcolm Farmer submitted a cite for "tri-di" from a reprint of James White's "Trouble With Emily"; Mike Christie verified the cite in the 1958 original magazine appearance. Fred Galvin submitted a 1953 cite for "tri-di" from an article by Pat Jones in Startling Stories. Mike Christie submitted a 1964 cite for "tri-di" from Harry Harrison's "The Starsloggers". Enoch Forrester submitted a cite for "tri-di" from a reprint of James Blish's "Earthman, Come Home"; Mike Christie verified the cite in the original 1950 magazine appearance.

Malcolm Farmer submitted a cite for "tridi" from a reprint of James Tiptree's "Parimutuel Planet"; Mike Christie verified the cite in the 1969 first magazine appearance. Dan Tilque submitted a cite for "tridi" from a reprint of Philip Jose Farmer's "A Few Miles"; Mike Christie verified it in the 1960 magazine appearance. Malcolm Farmer submitted a 1966 cite for "tridi" from Kenneth Bulmer's "Draft Dodger". Malcolm Farmer submitted a 1966 cite for "tridi" from Fred Saberhagen's "Mr. Jester"

Mike Christie submitted a 1947 cite from Lewis Padgett's "Tomorrow and Tomorrow", but it appears to be an abbreviation for "three-dimensional" rather than a reference to a broadcasting system analogous to TV. We would still like to antedate 1950.