When television networks had distinctive personalities something that hasn't been true for at least a decade now CBS was a classy dame, NBC was the network in the Brooks Brothers suit, and ABC was a raffish character, the likable underdog. Leonard Goldenson, chairman of the American Broadcasting Co. for 32 years, exhibits a personality similar to his network's in Beating the Odds, his corporate memoir. Goldenson's story is the tale of a third-place network that became a contender by pursuing two divergent strategies: beefing up its news coverage and dumbing down its entertainment programming. For every Nightline, there was a Love Boat footing the bill. Although Beating the Odds carries the exhausting subtitle ''The Untold Story Behind the Rise of ABC: The Stars, Struggles and Egos That Transformed Network Television,'' Goldenson is debilitatingly discreet. Here's an entire anecdote: ''[Sammy Davis, Jr.] was probably the most gifted performer of his era, he could sing, dance, play instruments, do standup comedy everything. And he was eager to go into television. We built a pilot around Sammy's family [in the '50s].... But no advertiser dared to back a 'colored' star at the time. We couldn't sell it, so 'The Sammy Davis, Jr., Show' never went on the air.'' Not very illuminating, is it? But it is typical, and that's too bad, because Goldenson seems to have been a smart, enthusiastic executive who really loved the idea of TV and its possibilities. Beating the Odds could have used more of his gambler's instincts.