Soul Train
Just laugh.
While I guarantee @Smooth Criminal will have some great thoughts, here's my quick rundown.This feels a bit awkward to ask but I as a player come asking for advice on how to play against D3. From a few times I've lurked over here, I hear some talk about my character being one of your absolute worst matchups and tScAtt (best-placing Mega Man currently) puts D3 as +2 in our list of best MUs. And yet, there's this really good D3 my region has who often consistently places at least top 15 or 10 while barely relying on a secondary: I had to play him at a local sometime last week; I got 2-0'd and sent into losers. Played him again at another 2 days ago, I managed to take a game off him this time but I was still defeated, and later lost to him once more (though I used that time instead, but still felt rough all the same). I keep telling myself that this shouldn't be as hard as it is, but That Bair... those Gordo Ledge Traps... thatt, Bair... his damage output... that Bair... and it feels so hard to kill him due to his weight, plus he seems nearly impossible to edgeguard from his D3 Jump. I do think I'm slowly beginning to read his tendencies, but it still feels difficult dealing with these things and as good as he is, I leave myself scratching my head as to how I keep losing a matchup that should be simple.
I do feel a bit embarassed asking this knowing this and to be having such a bad time with a matchup like this. But if humiliation is the key, I'll do whatever it takes to end my lose streak against him and I'd appreciate whatever help I can get.
Don't be embarrassed! The entire reason D3 wins games is because people don't know how to play against him, and you're doing exactly what you should to change that.
So here's the short on the matchup: Megaman is an incredibly safe zoner who forces opponents to approach, and D3 has no safe approach. None. Combined with horrible air and groundspeed, and D3 is pretty much running into projectiles the entire match. Gordos are a huge commitment for us (>60 frames!), but are easily reflected by lemons/gear (almost no commitment for you). You can outrun, outcamp, and even out-offense us. How? Ask yourself:
Is D3 on the ground? Yes: he has literally two options - shield, or slow ground attack (jab/ftilt). Since you get to dictate spacing in this matchup, all of these are beaten by your lemons/gear into mixups (grab, slide ->uair, etc).
Is D3 in the air? Great! Due to his awful airspeed and huge commitment to anything he does from the air, you're still at a great advantage. D3's either going to Bair or fastfall Inhale/grab. Both hugely leave him open, and have a huge windup to let you see it coming. Try to learn his patterns in the air - most D3s tend to have one or two air mixups they like - one of which is usually jump jump fastfall Bair. Learn the range of Bair, sit right outside it, and punish. The second D3 lands from ANYTHING, he's punishable.
Even if you get hit a lot - once you're above 80%, D3 can't do anything to you on shield. Our grabs won't kill til 190%+, and Inhale is just lol. To kill you, D3 is forced to make a big commitment with Dsmash. Which is hugely punishable by...shield. So pester with projectiles safely, wait for the opening, and punish (your Utilt WRECKS us if you manage to shield something at close range).
With Gordo edgetraps - they're really not as scary as you think. You can often just wait a bit, then falloff instant jump to lemons which reflect Gordo and beat almost everything we have. Just don't think you have to choose a ledge getup option immediately - most D3s charge Dsmash here, which is actually really hard to land. Mix up your timings, and you should be fine.
Just don't play at D3's Ftilt range - which should be easy due to your far better movement speed - and you're fine.
So play defensive. Be annoying. Dictate the spacing in the match. Force D3 to approach, and punish whatever he does since he has the worst approach in the game. Repeat.