Who is the only person (I think) that has hosted 3 different regularly scheduled TV programs on the 3 biggest networks simultaneously?

Alex Trebek I Beleive.

That is not the person I have. What programs did he host?

You know, that was my guess and he is recorded as being the only person in 1991 to host 3 daily game shows at the same time. However, the 3 shows ("To Tell The Truth", "Jeopardy!", and "Classic Concentration") were not shown on 3 different networks. "To Tell The Truth" was on NBC (at the time he hosted it) as well as "Classic Concentration". "Jeopardy!" was in syndication.

Hosting Truth made Alex Trebek the first and as of yet only person to host 3 national (American) game shows simultaneously, as he was also hosting Classic Concentration on NBC and Jeopardy! in syndication. (If other countries' shows are counted, Jim Perry joins Trebek on the list- while hosting the Canadian shows Definition and Headline Hunters in 1978, he also took the reigns of the first Card Sharks.)

I took a chance because the source doesn't mention the three different networks.

The answer I have is Dick Clark.

From Wikipedia -
For a period of several years in the 1980's Clark simultaneously hosted regular programmes on the 3 major American networks: ABC ("Bandstand"), CBS ("Pyramid") and NBC ("Bloopers"). He is the only person to have ever done this.

I checked on the dates and networks with the Internet Movie Database, and that part of the statement is correct. I will check on Trebek.
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Alex Trebek hosting credits during that time on imdb.com

"Super Jeopardy!" (1990) (No specific dates or network given)(Wikipedia says ABC in the summer of 1990)

"To Tell the Truth" (1990) TV Series .... Himself/Host (Feb. 4-May 31, 1991) NBC

"Classic Concentration" (1987) [TV-Series 1987-1991] (Wikipedia gives the dates as May 4, 1987 to September 20, 1991, and the network as NBC)

And, of course, "Jeopardy!" (1984) [TV-Series 1984-????] This is a syndicated series, so it may be on a different network in different cities. Here, it is on NBC.
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I see no mention of CBS in Trebek's credits above. I guess Trebek qualifies as an answer if he was on CBS somewhere at that time. I should have put "nationally broadcast" in the question.