The Wildlife Protection Act, 1972 is an important legislation in India that aims to protect and conserve wildlife and their habitats. It categorizes different species of animals and plants under various schedules, depending on their conservation status and the level of protection they require. Schedule 3 of the Wildlife Protection Act includes certain species that are protected to some extent but are not as critically endangered or threatened as those listed in Schedule 1 and Schedule 2.

Among the given options, the animal that is not listed under Schedule 3 of the Wildlife Protection Act, 1972 is the Sloth bear. Let's understand why this is the correct answer.

Explanation:
- The Wildlife Protection Act, 1972 has six schedules, each containing a list of various species of plants and animals.
- The animals listed under Schedule 1 are considered highly endangered and receive the highest level of protection. Offenses related to these animals can result in severe penalties.
- Schedule 2 includes species that are not as critically endangered as those in Schedule 1 but still require significant protection.
- Schedule 3 includes species that are not as threatened as those in Schedule 1 and 2 but still require some level of protection.
- Nilgai, Hyaena, and Barking deer are all listed under Schedule 3 of the Wildlife Protection Act, 1972, which means they are protected to some extent.
- However, the Sloth bear is not listed under Schedule 3. It does not mean that the Sloth bear has no protection at all, but it falls under a different schedule or category.
- The Sloth bear is actually listed under Schedule 1 of the Wildlife Protection Act, 1972, which means it is considered highly endangered and receives the highest level of protection.
- This categorization highlights the critical status of the Sloth bear and the need for stringent measures to conserve and protect this species.

In conclusion, among the given options, the animal that is not listed under Schedule 3 of the Wildlife Protection Act, 1972 is the Sloth bear. It is actually listed under Schedule 1, indicating its highly endangered status and the need for strong protection measures.