Three Piper J3 Cubs departed an airport in a valley with higher terrain nearby, and a density altitude of about 3,000 feet. Each airplane turned toward rising terrain, consisting of lower finger ridges and canyons, and a higher main ridge. The first airplane carrying only a pilot climbed faster than the terrain, and turned downwind. The second and third airplanes each carried a pilot and passenger and tried to follow similar paths. The pilot of the third airplane, which was operated about 100 pounds (9%) over gross weight, reported he watched the second airplane descend into trees. After overflying the second airplane, he encountered descending air which drove him into the trees. The pilot reported no problems with the airplane or engine. The pilots were briefed on alternate departure routes that would not require an immediate climb over rising terrain, but they were not used. FAA publications advised of the need to download weight when operating with higher density altitudes, and to approach ridgelines at a 45 degree angle to allow for escape in case of turbulence or descending air.