I "main" d3, try nairing where they are(facing the direction they are going to roll) and then buffer a dtilt when you land so when they roll out of the nair you hit them with the dtilt
ALAKAZAM you can punish rolls
I don't know who you're fighting that doesn't punish your nair with a shieldgrab.
Nair doesn't exactly have the best range, not to mention if anyone even touches you, they knock you out of it.
I used to think D3 was a great character until I found out that everyone and their mother has decent options vs him; he does great with controlling space provided your opponent is cornered, but his options for general stage control are bad and landing is a pain due to the fact that he needs to either be in prime position for an autocancel bair, preparing to bait a usmash/utilt, or just not be anywhere near his opponent.
As far as punishing rolls, like I said, it's not like I'm not ever getting the punishes, I just feel it's more tedious than it needs to be; it's a lot less "Wow, it sure is easy to punish this option" and more of "I knew you were going to use this thing, so I've got a punish prepared".
I'd appreciate it if you'd actually explain why what you say works and give multiple options rather than giving me a perceived solution to one scenario and saying it's done; as an actual D3 main, I know that dtilt has ridiculous priority and can clash or overpower quite a few things, but if you hit shield (which you most likely will), it's a braindead punish because of how much endlag it has, so it's generally safe to not use it unless you're guaranteed a punish -- if you're fighting a player that's scared of you, then yes, they will roll back, but most of the time, people are confident enough in shield that they'll just grab you or jab "lock" you in the case of Fox.
Long story short, it's nowhere near impossible to deal with rolls because I've managed to do it thus far, even against ROBs and Rosalinas, but it's a lot more annoying than past iterations and no amount of "gitting gud" is gonna change that.
It's actually more boring than fun/"rewarding" to play a flowchart match; if I already know almost everything the person is gonna do but all I have to do is time my stuff right, it just gets pointless.
Even otherwise solid players tend to fall into the same freaking habits because the game is becoming more barebones paper-rock-scissors like traditional fighters.
It might feel good for you to stomp people over and over again in similar ways, but for me, it lacks intricacy and general enticement, and I'll be happy when we find something cool and new in the game.