Common Grounds

[ReKoil]

Hmmm.... not all steel types due to Gallade.
Also not all even PokeDex numbers as Gallade is # 475.
If not for Gallade it could've been a roughly even gender ratio.
No shared abilities or stats.... evolution methods... base stat totals are also not equal....
Except for Corviknight they can all learn Slash... but it's listed as bonus so it should have the commonality too.
They can all learn Fury Cutter... except for Corviknight.
The amount of stages in the line? No, if I count split evos the Ralts line has 4, and the others 3.

Well it's probably not what you meant, but I'm stumped so I'm just going with this:
All of them can learn Protect via TM.

Edit: lol, it's probably when they were introduced... even generations! Gen2, Gen 4, Gen 6, Gen 8!

Diggersby, Hawlucha, Croagunk
Hint: Type combination related.
Hint 2: Something they lost.
Giving two hints as I think it might be a hard one.

 

[CF1994]

1. The first Pokémon discovered with unique type combinations at the time.
2. The ability to learn Return is lost due to the move getting Thanos'd.

Farfetch'd, Corsola, Linoone.
Hint: They were well overdue for this.

 

[PokeDude645]

An evolution! :D


Beeheeyem, Drednaw, Spiritomb

Hint: Numbers

 

[ReKoil]

Well it's not the number of letters...
The only thing their Dex numbers have in common is that they are even numbers... which could be it.
Oh... stats maybe?

They all got a Base Stat Total of 485.

Whitney, Norman, Lenora

 

[VisionofMilotic]

That's easy, they're all normal type experts!

Ho-oh, Slowbro, Cyclizar

 

[ReKoil]

They can all have Regenerator as their Hidden Ability.

Sprigatito, Applin, Pawniard.
Hint: Moves

 

[Simply Dunsparce]

They can all learn Sucker Punch!

Starly, Munchlax, Pachirisu
Hint: It involves a Pokemon Ranger game

 

[ReKoil]

You have to choose between them for your first Partner Pokemon in Pokemon Ranger: Shadows of Almia.

Slowpoke, Alolan Marowak, Rowlet
Hint: Isle of Armor