Before you decide to run off and join the circus, watch the acrobats rehearse in the solitude of the empty big top. Whether holding up the balancing pole or trying to fit one more somersault in before you hit the mat, circus work is a workout. Cirque du Soleil, the French Canadian troupe that opened the Los Angeles Theater Festival, is ending a three-month stay in Southern California Sunday. After opening to rave reviews in Little Tokyo, the circus traveled to San Diego and then set up its big top next to the Santa Monica Pier. Of the 75 performers and technicians in the troupe, about half live in trailers and mobile units on the premises. The Cirque's California (and U.S.) debut has been a sell-out, but the oceanfront site was not as idyllic as one would think. "The water was polluted and it's been cold," said Barbara Gosselin, who sells cotton candy and popcorn while her boyfriend does his acrobatic act. She extended a pale hand for inspection. "Look," she said, "after three months in California--no tan."