I have actually done testing and lots of lab work on the d3 edge guard game.
Nothing we have beats though his super armor, even at 999%, other than thunder (the thunder may have caught his 1 frame of ledge grab vulnerability, so this is actually questionable)
The only way to beat his super armor is to hit the 1 frame window on his ledge grab.
D3 does not have super armor during the startup of his UP B, so you have to start pressuring him from really low.
If D3 is coming from high or around stage level, try to push him away with your aerials and traps (Don't know the best way to do this)
If he recovers low and goes for the super safe UP B, Jump offstage with jolts, aiming the jolts for the soonest possible place that he could up B, then fair him once you get to him.
This is the only thing that I have found useful in edge guarding D3.
IMO, the options you have to edge guard D3 are as follows.
1. Play the ledge trump game (You can quick attack to instant snap the ledge, which makes this even better)
2. Respect his offstage game, his recover is VERY hard to deal with
3. Try to catch his 1 frame of ledge grab vulnerability with a long lasting hitbox like thunder (this is easy since his up B does not have a hitbox, and you can react to the startup on his UP B, you could use Bair, dair, nair, and jolt too)
4. Do the edge guard game that I listed above
Just remember that D3 can keep us on the ledge easier than we can. So we do not want to let him back to the stage while being offstage ourselves.