GLENDALE, Aug. 16— WP) Brush fires which have razed more than 200 acres in this area during the past two weeks were started by three juveniles who have confessed, Glendale Fire Marshal Al W. Hunt said today. An 11-year-old camper at Camp Max Strauss told Hunt he touched off the blaze that swept 150 acres and endangered 40 homes in the Whiting Woods section near here last Monday. A nine-year-old girl and a 10- year-old boy told officers how they watched the Aug. 6 fire at Eagle Rock which destroyed three homes, and then two days later set fire to the brush along El Roble Dr. That blaze swept 20 acres before it was brought under cow were atuittedly started by other youngsters, but none resulted in serious damage.