Rule 6.25 states, in part: "At least one unit must attack..."
Does this mean at least one unit from each hex marked for attack or at least one unit from any hex marked for attack? I assume the latter.
Attack markers are placed on top of friendly units. The rule addresses the matter of tossing down an attack marker and then doing nothing. If you place an attack marker on a stack of three friendly units at least one of them must attack.
You never put an attack marker on enemy units and you may never "choose" which enemy units in an enemy stack to attack. You must attack the entire stack.