Galumph, galumph. What ho? "Ivanhoe!" A ho of a different color, one might say, but not nearly different enough. The new three-hour CBS version of Sir Walter Scott's just-as-well-forgotten novel, airing tonight at 8 on Channel 9, is another of the network's agonizingly dull historical costume dramas, produced by the unfortunately undiscourageable Norman Rosemont.

Rosemont and CBS is a lethal combination, like double Dramamine with a Quaalude chaser. Rosemont ransacks the public domain for tired old tales viewers can be tricked into sitting through one more time; only once, with "All Quiet on the Western Front," has a Rosemont production really attained any semblance of vitality or flair.

Everything in television is relative to everything else in television, however, and viewers who might otherwise be watching ABC's asthmatic old sitcoms, or the tattered timekillers available on NBC, may find pleasantly irrelevant respite in "Ivanhoe," and the horses are pretty. If CBS weren't showing a three-hour "Ivanhoe," it would probably be showing a two-hour rape, so maybe we all owe Norman Rosemont a debt after all.

Anthony Andrews, a sensation as Sebastian in "Brideshead Revisited," plays Scott's hero in the film, but in the first hour he is hidden behind a jousting helmet most of the time. The jousting goes on and on and on--"mortal combat" on "noble steeds," as John Gay's high-school screenplay has it--and director Douglas Camfield keeps cutting to reaction shots of listless extras feigning interest.

Ivanhoe means well, but such a klutz. He is injured in the jousts, faints, and spends the entire second hour recuperating, carried about unconscious on a litter for a while, later locked in a tower with the still beautiful Olivia Hussey. Unfortunately, Ivanhoe, like most of the characters, is an anti-Semitic moron, and this is supposed to make "Ivanhoe" into a lesson on the evils of prejudice. It is more of an essay on the evils of dullness.

"Dogs! Norman dogs!" brays Michael Hordern, in a funny fright wig, as an idiot Saxon, Cedric of Rotherwood. Cedric? Of Rotherwood? Isn't this the kind of drivel that Monty Python sought to rid the world of once and for all? But here are Rosemont and Gay, who collaborated on the recent and even more laborious remake of "The Hunchback of Notre Dame," playing it with straight faces. "Come on," says the Black Knight, "let's put an end to this nonsense!" But neigh, when he saith it, there is yet another hour to go.