THE THIRD program ol the 1982-83 concert season of the William Appling Singers and Orchestra will be6resented Feb. 20 at 6:30 pm. in lurch Auditorium, the Cleveland Museum of Natural History in University Circle.The concert will include: “Ave Verum Corpus” and “Sinfonia Concer-tante” by Wolfgang A. Mozart and “Flos damp!” and “Serenade to Music” by R. Vaughan Williams.Reserved seats are $7.50 and $5, with group discounts available. For tickets calf 791-0643.The William Appling Singers and Orchestra, a group of professional musicians from Greater Cleveland, are in their fifth season of presenting a series of large chamber works of all periods and styles.Appling, who will conduct the program, is chairman of the music department at Western Reserve Academy in Hudson.