Do all Pokemon have to be present in a game to complete the Pokedex?

Question

If I have Zapdos can I trade for Moltres then Articuno and consider that done or do I need Zapdos, Moltres, and Articuno ingame? (In ORAS.)

Answer 1

No.

What you are referring to is called a living PokéDex. That is where you have every Pokémon present at one time, e.g. all 700+ in your PC at once.

To complete the National Dex, however, you simply need to have had one of each Pokémon at one time. So it's fine to trade your Zapdos. Also, to complete the National Dex you don't need any of the Event Legendaries.