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