The City Dance festival, now in its third year, has been marked from the start by a spirit not easily found elsewhere - a spirit compounded of fellowship, of mutual pride, of a sense of artists as part of and an expression of, a particular place and itme. All this was in evidence as "City Dance '79" was launched at the Warner Theater last night, with the presentation of the first three of 10 area dance troupes.

But it was also clear how far removed the event is from being merely a cozy municipal get-together. The dancing throughout the evening was first-rate, and the program would have shown brightly against any conceivable background.

Part of the excitement is generated by the Warner itself, whose location and ambiance make one feel this is truly a "city" event, and whose stage is extremely flattering to dance performance.

A case in point was the Jan Van Dyke and Dancers troupe, which led off the program last night with Van Dyke's "Variations on a Theme." The piece strives to show how a basic combination of low-key steps and phrases-the "theme" - can take on diverse colorations from varied musical settings, ranging from baroque to disco. Though the work is more compelling in concept than in actualization, the performance was impeccable - this smooth-limbed company has never looked better, and that goes for Van Dyke herself, also seen in a sassy solo from her "Fleetwood Mac Suite."