Detailed Explanation:

Sea Cow (Dugong): A herbivorous marine mammal found in coastal waters of the Indian and Pacific Oceans; classified under class Mammalia.

Sea Horse: A carnivorous fish (not a mammal) belonging to class Osteichthyes; found in shallow tropical and temperate waters.

Sea Lion: A carnivorous marine mammal of family Otariidae; breeds on Pacific and Atlantic coasts; classified under class Mammalia.

Therefore, only Sea Cow (1) and Sea Lion (3) are mammals.