From Bulbapedia, the community-driven Pokémon encyclopedia.
The Water 3 Egg Group (Japanese: 水中③グループ Underwater 3 Group, formatted as すいちゅう3 Underwater 3 in Pokédex 3D and Pokédex 3D Pro, 水中グループ3 Underwater Group 3 in Generation III and IV guidebooks, and 水中3グループ Underwater 3 Group in Pokémon Stadium 2 and Generation II guidebooks) is one of the fifteen Egg Groups. Like its counterparts Water 1 and Water 2, it is mostly made up of Water-type Pokémon. As of Generation IX, 37 Pokémon belong to this group.
Characteristics
The Water 3 Egg Group is mostly characterized by its members' resemblance to aquatic invertebrates; most are Water types. This group also includes several Fossil Pokémon, totaling a little more than half of those which are capable of breeding.
It is a relatively small Egg Group. Most Pokémon that are capable of breeding into other Egg Groups breed into the Water 1 Group.
Pokémon
Only in this Egg Group
In this and another Egg Group
Trivia
In other languages