Is it only xerneas and yveltal or is it xerneas yveltal zygarde?

Question

is it a duo or trio legendary group?

Answer 1

Xerneas, Yveltal and Zygarde are a trio.

Xerneas – The Life Pokémon. Represents extreme good.

Yveltal – The Destruction Pokémon. Represents extreme evil.

Zygarde – The Order Pokémon. Balances the two extremes represented by Xerneas and Yveltal, because too much of one would end in disaster.

Answer 2

Yes they are a trio. Infact, they are led by Zygarde.
Source

Answer 3

They are a Mascot Trio (X, Y, Z) Like Palkkia Dialga andGirintina or Reshiram Zekrom and Kyurem or Kyogre Groudon and Rayquaza.

Answer 4

They are a trio, yet zygarde, this so called leader, is the weakest of the three. But yes, they are a trio. Hope I helped!