Common Grounds

[Orion☆]

I don't watch the Anime so I was kinda waiting for someone else to get it =p
All three of them were part Ghost type, but that's not specific to the Anime hint.

Aight, aight, since no one could solve it, I'll just clarify it myself. Yes, they're all Ghost-types, and all of them were owned by Jessie of Team Rocket.

As for your new riddle, all three of them have Hisuian forms in Legends!

Now, for a new one: Kecleon, Greninja, Cinderace. Hint: ability.

 

[ReKoil]

They all have an ability that lets them change their type to that of the move they are about to use (Protean and Libero).

Pidgeot, Altaria, Abomasnow.
Hint: Gen 6

 

[Megan]

They got better when they got their Mega evolutions in gen 6. Probably some of the few Pokemon who actually deserved one, as well. :P

Pinsir, Heracross, Jynx
Hint: Scyther doen't belong

 

[Orion☆]

Uhh... the fact they can't evolve any further (barring Mega Evolution) whereas Scyther can (into Scizor and Kleavor), perhaps? The consequence being that they belong to a group of Pokémon that do not benefit from the Eviolite.

Gastrodon, Chandelure, Muk.
Hint: Egg group

 

[Simply Dunsparce]

They're all in the Amorphous egg group

Machamp, Beartic, Copperajah
Hint: Base Stats

 

[Fairy]

They all have 130 base Attack stats?

Skiploom, Foongus, Flapple
Hint: It's not their Grass typing.

 

[ReKoil]

They all weigh the same (1.0kg) ?

Kyurem, Necrozma, Calyrex

 

[Orion☆]

All of them can be fused with either one of two Legendary Pokémon to recover their full power - Kyurem with Reshiram or Zekrom, Necrozma with Solgaleo or Lunala, and Calyrex with Glastrier or Spectrier.

Obstagoon, Buzzwole, Golurk
Hint: Designer

 

[ReKoil]

Well guess I'll be the one to look these up on bulbapedia lol
They were all designed by James Turner

Murkrow, Lampent, Doublade
Hint: Evolution method

 

[Crimson the Cowboy]

Easy, dusk stone.

Gliscor, Shuckle, Electrike

 

[ReKoil]

Well... they all have a blue and yellow shiny, although the color tones across the three differ a bit. But it's all I can think of =p

Emolga, Pachirisu, Morpeko

 

[Aquacorde]

Electric rodents, must be!

Absorb Bulb, Growth, Petaya Berry!

 

[Eleanor]

They all increase Special Attack when used or consumed~

Aeroblast, Strange Steam, King's Shield

 

[ReKoil]

They're all signature moves.

Will, Lucian, Caitlin

 

[Orion☆]

They're all Psychic-type specialists who featured somewhere in the Gen IV games. (Caitlin, however, was not battleable until Gen V.)

Silver, Anabel, Marley.
Hint: A Pokémon - figure out which!

 

[Citra La Magicienne]

They all use Weavile at some point-


Aurorus, Ninetales, and Politoed.
Hint- abilities!

 

[ReKoil]

They all can have an ability that influences the weather: Snow Warning, Drought, Drizzle.
These all also happen to be their HA's.

Lurantis, Mimikyu, Kommo-o
Hint: Gen 7

 

[Eleanor]

They're all Totem Pokémon in the Gen 7 games (and introduced in Gen 7 themselves ofc)!

Veilstone City, Lilycove City, Goldenrod City
hint: building~

 

[ReKoil]

The Department store!

Strength, Rock Smash, Waterfall

 

[PokeDude645]

They are all Hidden Moves!

Scizor, Gallade, Aegislash, (bonus) Corviknight
Hint: Even