Odd one out!

[strangerhypno]

Sliggoo evolves from Goomy at level 40, other two evolve from 30

Ampharos, Tyranitar, Sceptile

 

[正真正銘]

I'm guessing Tyranitar because his Mega isn't part Dragon.

Qwilfish, Tentacruel, Mareanie

 

[blue]

Qwilfish is the only Pokemon that isn't part of an evolutionary line.

Skarmory, Swellow, Pelipper

 

[CF1994]

Skarmory was not introduced in Generation III. It also does not have a pre-evolution.

Steenee, Gothorita, Kirlia

 

[正真正銘]

Steenee is a Grass type while the others are Psychic type.

Raichu, Raikou, Darkrai

 

[Juno]

Raichu is the only one with an Alolan form

Audino, Miltank, Kangaskhan

 

[VisionofMilotic]

There are quite a few things I can think of, but what's most apparent to me is the gender differences. Audino is odd because it can be either male or female. Miltank and Kangaskhan are exclusively female.

Gengar, Mismagius, Spiritomb?

 

[CF1994]

Spiritomb is not part of any evolution line. It also cannot casually be found in the wild unless you complete some specific steps to find one.

Probopass, Stakataka, Bastiodon

 

[正真正銘]

Stakataka doesn't have the ability Sturdy nor was it introduced in Generation IV.

Gyarados, Pelipper, Mantine

 

[CF1994]

Mantine was not introduced at the same time as it's pre-evolution, Mantyke. The pre-evolution of Mantine cannot breed, whereas Gyarados and Pelipper's pre-evolutions can.

Ninetales, Sandslash, Dugtrio

 

[正真正銘]

Sandslash's name doesn't contain a number. Or Ninetails isn't a Ground type.

Abomasnow, Vanilluxe, Mamoswine

 

[VisionofMilotic]

Vanilluxe is pure ice right? The others are dual types.

Hawlucha, Lucario, Gallade

 

[CF1994]

Hawlucha has no pre-evolution and Mega Evolution.

Electivire, Ampharos, Xurkitree

 

[miragespot]

Xurkitree is an Ultra Beast and has no pre-evolutions.

Delibird, Jynx, Piloswine

 

[Arma]

Jynx isn't a Pokemon from the second generation.


Golduck, Floatzel or Clawitzer?

 

[CF1994]

Clawitzer is not part of the Field Egg Group.

Aurorus, Lapras, Abomasnow

 

[miragespot]

Lapras does not have a pre-evolution. (It should though!)

Eevee, Farfetch'd, Girafarig

 

[faf]

Farfetch'd's name isn't a palindrome.
Groudon, Xerneas, Necrozma

 

[Ace Trainer 188]

I'm sure the last one is a mascot of a third version/sequel.(or I guess both for this case) while the other 2 are exclusives to the first set of games in a generation.

Meganium, Vileplume, Abomasnow

 

[Hyzenthlay]

Meganium doesn't have a dual typing!

Mawile, Zangoose, Spinda