Moltres, Articuno, Zapdos

Hey I want to know, what happens if you kill one of the legendary birds in FireRed. I have caught Articuno using the Master Ball I got at Saffron. But when I met moltres I used all my ultra balls and it didn't work. So I just killed it and it dissapeared. Will I never see it again? How am I going to complete the pokedex??