A psychedelic happening awaits those are planning to visit the Atlantic City area in early August. The Atlantic City Race Track in nearby Mays Landing, N.J., will play host to a three-day marathon concert performed by headline entertainers from the poo and folk-music worlds. The Atlantic City Pop Festival will start at 2 P.M. on Aug. 1 and continue until midnight. On Aug. 2 and 3, the concerts begin at noon and last until midnight. Tickets at the door cost $6.75 a day, but by writing to the race track for reservations, the full three-day ticket is available for $15. The entertainers will include Janis Joplin, Jefferson Airplane, Joni Mitchell, Procol Harum, Creedence Clearwater and Little Richard.