Unocal Corp. has made three executive appointments in its petroleum exploration and production operations. Harry C. Lee, who joined Unocal as a geologist in 1959, was named executive vice president-international gas and oil. Lee previously was vice president of Unocal’s Gulf region. Graydon H. Laughbaum Jr., who has been vice president of Unocal’s central region, will assume Lee’s position in the Gulf region. Martin F. Miller, who has been manager of the central region, has been promoted to vice president of that operation.