- There is a cat, a dog, and an elephant in the room.
- There are two cats, a dog, and an elephant in the room.
- There is a cat, two dogs, and an elephant in the room.
In the first sentence, all of these animals are acting as a single unit, thus the subject is singular. But I'm not sure about the 2 and 3. Why plural subject for 2 and the opposite for 3?