Which three of these classes of animals practice internal fertilization?

1) mammals
2) birds
3) amphibians
4) reptiles
5) arthropods

The mammals, birds and reptiles are the three classes in the animal kingdom that practice the internal fertilization. These three classes, that is the birds, mammals, and the amphibians, always use fertilizes the egg internally. The amphibians on the other hand fertilizes externally. The arthropods, on the other hand, reproduce both by external fertilization and internal fertilization.

Hence, the answer is 'mammals, birds, and reptiles'.

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