How do you get the three Regis after beating E4 and the champion in Sapphire?

Question

So I accidentally killed / made the three Regis faint before beating E4 and I heard that you could get them again after beating the E4. I went to check if the Regis were there and they weren't.. If you could help, that'd be very much appreciated.

Answer 1

In games older than Platinum, legendary Pokemon disappear forever when they faint.
source
From now on, remember to not save the game after doing anything dumb, such as killing a legendary Pokemon.