AUTHOR GRANTED THIRD DIVORCERichard LeRoy Stokes, Washington, D. C., Journalist and playwright, today ended his third marriage when he divorced Kathryn Belle Stokes on a charge of extreme cruelty.The court granted Mr*. Stokes $200 monthly alimony to he paid until remarriage or death but reserved jurisdiction, to change the amount.The couple married In Baltimore, Wd., In 1986 and had no children.Stokes, who recently had been in New Mexico doing research for a series of articles on the atomic bomb, has been a member of the Washington bureau of the St. Louis Post-Dispatch since 1987.