What term describes insects that develop in three stages: egg, nymph, and adult?

The correct term for insects that develop in three stages—egg, nymph, and adult—is gradual metamorphosis. Insects that undergo this type of development do not have a distinct larval stage, as seen in complete metamorphosis. Instead, they transition from an egg to a nymph, which resembles a smaller version of the adult, and then to the adult stage. This process allows the nymphs to gradually acquire adult characteristics over several molts without a drastic change in form, unlike what is seen in insects with complete metamorphosis. Thus, the use of the term gradual metamorphosis appropriately describes this developmental process.