Western Digital, an Irvine-based computer products company, has promoted three executives--A.J. (Bert) Moyer, Kathy A. Braun and John R. MacKay--to senior vice presidents. They will report to company Chairman Roger W. Johnson. Moyer, 44, will retain his position as chief financial officer, a post he has held since joining the firm in February, 1986. Braun, 36, will continue as general manager of the storage products unit; she joined the company in December, 1978. MacKay, 41, who joined the company in January, 1983, will continue as head of worldwide manufacturing.