Odd One Out

[Nah]

Ampharos is the only one not used by Cynthia.

Voltorb, Snorlax, Wooper

 

[Apollo]

Wooper wasn't a part of a main story..? (Like the Voltorb at the Rocket Hideout in Johto, and the Snorlax blocking your path.)

Seaking, Clefable, Honchkrow

 

[Pryze]

Seaking and Clefable are both from Kanto, while Honch is from Sinnoh.

Volcarona, Hawlucha, Rotom

 

[destinedjagold]

Rotom has different forms! ^^

Skitty, Meowth, and Purrloin~

 

[PieInABox]

Magneton, it's the only one based off an inanimate object!

Cacnea, Meloetta, Beldum!

 

[Hikamaru]

Electrike's the only one that isn't rodent-based.

Okay, this one:

Mega Lucario, Mega Gengar, Mega Ampharos

 

[imas234]

mega gengar didn't get more fabulous as he mega evolved i.e cool hair/hair like stuff
He's also the only one with a third eye.

Ponyta, Vulpix, Growlithe

 

[bunny24680]

Ponyta...........It's the only one that doesn't need Fire stone to evolve!!!

Pidgey, Charizard and Scyther!!!

 

[Trick 'r Treat]

Scyther, it's the only one that doesn't have its final evolution at lvl. 36

Gardevoir, Kecleon, Jellicent

 

[Nah]

Gardevoir is the only one with a Mega Evolution.

Zekrom, Jellicent, Krookodile

 

[classiccartoonsftw]

Zekrom wasn't used by an Elite Four member. And no, N doesn't count!

Dewgong, Cloyster, and Lapras

 

[Trick 'r Treat]

Medicham is the only one with total base stats not in the 500's

Hypno, Altaria, Magcargo

 

[imas234]

Hypno because it can't learn fire blast

Dodrio, Pidgeot, Unfezant

 

[PieInABox]

Dodrio, it's the only one with 3 heads!

Torkoal, Clefable, and Chesnaught.

 

[imas234]

Torkoal is the only one who walks on four legs

Torkoal, Carracosta, Torterra (something other than types please)

 

[PieInABox]

Torterra - It was introduced in an even numbered generation!

Lugia, Staraptor, and Heatran!

 

[bunny24680]

Darmanitan as till now, only he has forms but delphox and victini doesn't......

other way is Victini as it is the only legendary amost the three

Now here is a hard one:
Blastoise, Samurott and Feraligatr

 

[Candy]

Feraligatr evolved from Croconaw at min lv 30, while the other two evolved at min lv 36. At least that's what I know

Dragonair, Sliggoo and Shelgon

 

[PieInABox]

Sliggoo is the only one that looks like it evolves into it's final form!

Magneton, Haxorus, Cofagrigus.

 

[Sushimmi]

Blaziken
It doesn't have flames on its body. Not including Mega.

Jolteon, Shiftry, Delcatty

Edit: Is it too hard? (Laugh)