High concentrations of two hazardous solvents have been found in ground water under three Burbank plants of the Lockheed-California Co., according to a study filed by the aerospace firm with state water quality officials. The study provides the strongest evidence yet that Lockheed contributed to contamination that has forced Burbank and Los Angeles to shut down more than 30 municipal water wells in North Hollywood and Burbank. A company official insisted, however, that other firms may have caused the pollution. About 15% of Los Angeles' drinking water is drawn from San Fernando Valley wells and is piped primarily to central and southern Los Angeles. The industrial solvents that began showing up in water from the wells in 1980 are trichloroethylene (TCE) and perchloroethylene (PCE).