post_parent

hi at all, i have a little problem,
$myposts = get_posts('post_parent=1&post_type=page&orderby=rand&numberposts=1');

with this code i display a random subpage of a mother page, all OK, but if I put another mother page
$myposts = get_posts('post_parent=1,2,3&post_type=page&orderby=rand&numberposts=1');
nothing, the code display a casual page of other mother page…

where is the mistake?
tank at all
kikko088