A comparison minus variations of enemies, and only concerning the vanilla game.
Fallout 3-
Raiders, Super Mutants, Centaurs, Bloatflies, Mirelurks, Deathclaws, Ants, Yao Guai, Feral Ghouls, Protectrons, Robobrains, Sentry Bots, Enclave Forces, radscorpions, Talon Company, Mr. Handy's, Mr. Gutsy's, molerats
Total: 18
Fallous New Vegas-
Raiders (three to four tribes of them), Deathclaws, Cazadores, Bloatflies (all five of them), Super Mutants, Centaurs, Protectrons, Securitrons, Sentry Bots, Feral Ghouls, moss fungus beasts, mantis's, racscorpions, ants, Mr. Handy's, the snake dogs, coyote's, gecko's, molerats
Total: 19
If you wish to account the possible factions that can become your enemy: 23
Fallout 4-
Raiders, Feral Ghouls, Sentry Bots, Synths, bloatflies, mutant dragonflies, mutant mosquitos, super mutants, super mutant hounds, Mirelurks, Deathclaws, Gunners, radscorpions, Mr. Handy's, Mr. Gutsy's, Assaultrons, dogs, molerats
Total: 18
If you wish to account the possible factions that can become your enemy: 22
I may have missed one or two types.