Water 3 egg group

37 Pokemon belong to the Water 3 egg group in Cobblemon. Any two of these that are opposite genders can breed together, and Ditto works with all of them.

Tentacool waterpoison #0073

Tentacruel waterpoison #0090

Shellder water #0091

Cloyster waterice #0098

Krabby water #0099

Kingler water #0120

Staryu water #0121

Starmie waterpsychic #0138

Omanyte rockwater #0139

Omastar rockwater #0140

Kabuto rockwater #0141

Kabutops rockwater #0222

Corsola waterrock #0341

Corphish water #0342

Crawdaunt waterdark #0345

Lileep rockgrass #0346

Cradily rockgrass #0347

Anorith rockbug #0348

Armaldo rockbug #0451

Skorupi poisonbug #0452

Drapion poisondark #0564

Tirtouga waterrock #0565

Carracosta waterrock #0566

Archen rockflying #0567

Archeops rockflying #0688

Binacle rockwater #0689

Barbaracle rockwater #0692

Clauncher water #0693

Clawitzer water #0739

Crabrawler fighting #0740

Crabominable fightingice #0767

Wimpod bugwater #0768

Golisopod bugwater #0864

Cursola ghost #0950

Klawf rock #0960

Wiglett water #0961

Breeding within Water 3

Sharing an egg group is only the first requirement. To actually get the Pokemon you want:

Put both parents in a pasture block and claim the land it sits on.
Give one parent a Destiny Knot so five IVs pass down instead of three.
Give the other an Everstone carrying the nature you want.
Make sure the father knows any egg moves you need, since those pass down too.
Replace parents with better offspring as you go, and expect many eggs.

Full walkthrough in breeding on TalonMC.

Other egg groups

Field No Eggs Discovered Water 1 Bug Grass Mineral Monster Flying Dragon Human-Like Fairy Amorphous Water 3 Water 2 Ditto