If it's an official Video Game Championship (VGC) event, then I am afraid that none of the Pokémon, Legendary or otherwise, from Sapphire will be allowed. There's a rule called the Blue Pentagon Rule, which means that if a Pokémon was not hatched/caught/received in either XY or ORAS, then it cannot participate in an official event.
Source: Experience
However!
If using an Alpha Sapphire game, or any Gen VI game at that, then any and all Legendaries that can be caught in those game are legal with the exception of Deoxys. You may also have only two of the following on your team:
>Mewtwo, Lugia, Ho-Oh, Groudon, Kyogre, Rayquaza, Dialga, Palkia, Giratina, Reshiram, Zekrom, Kyurem (normal, White and Black fusions), Xerneas, Yveltal, Zygarde
