A former Army medical instructor was sentenced to five months in the stockade and a dishonorable discharge for having sex with three women soldiers without telling them he carried the AIDS virus. Sgt. Richard W. Sargeant, 28, also was demoted to the lowest rank of private and stripped of all his military benefits. In military court at Ft. Sam Houston in Texas earlier in the day, Sargeant had pleaded guilty to one count of sodomy, two of adultery and two of disobeying an officer who warned Sargeant to wear a condom or tell prospective sexual partners of the AIDS virus. The jury agreed Sargeant should be confined for nine years, but a plea bargain approved before the punishment phase called for the shorter term.