Ok, well I decided to write a Marth summary in case you wanted to use it. If you don't want to use it thats ok
I took it all from what people wrote with the discussion, with a tiny bit of personal experience.
Marth Summary
This matchup can be easy if you know how to handle marths but can be difficult otherwise. Marth has a terrible recovery, and you can abuse this by CGing him to the edge and attempting to gimp him with bairs. Although his UP+B has good priority so watch out for that. Also notice that sometimes his UP+B will hit you if you are standing on the ledge and he UP+B's into the ledge. The marth's spacing has to be near perfect or he will get easily punished by DDD and his large grab range, while he has trouble punishing DDD for moves. Ftilt works great to stop his spaced wall of Fairs and Nairs and his Dancing blade. Shield grab his Dancing Blade. Most of the time if they see they are going to get grabbed they will stop the dancing blade on the 1st or 3rd swing, but it is still possible to grab them, although beware the down (green) version can shield poke if your shield is already low.
Also note that Marth has a chaingrab on DDD with fthrow up to around 150% (not sure, this is what Gates wrote in the Marth discussion but I think you can easily get out of it by then, can someone confirm). At 0% he can Fthrow-> tippered Fsmash and he can spike you out of the chaingrab until around 12% However DDD gets thrown a fair way forward so Marth can only get around 6-8 grabs in on FD assuming he starts at the end, which is around 24%. Although Ftilt is great for stopping his approaches it won't stop all of them so don't rely on it or spam it. You're going to have to SCG well this matchup or marth can Up+B out of it. If you predict it however you can upsmash him while his in his helpless state, or regrab and Dthrow for more mindgames. Marth is the lightest character you can chaingrab so at higher percents if you predict the UP+B, Utilt or Usmash it, or pull out the Jet hammer.
As mentioned before a great way to kill Marth is by gimping him with wall of pain Bairs and then edgehogging to make sure. You don't even have to send him out that far to kill him and the chaingrab gives you an opportunity to get him offstage.
A common move for Marths to pressure shields is SH double Fair -> Dancing Blade down version (green). This will punish you if you attempt to grab after the Double Fair. You can also outcamp Marth but remember that he is fast and if you throw a waddle dee when he his too close he can punish you for it. I doubt marth could get you far enough off the stage to be force you to use your UP+B because of his bad recovery but when you must use it be careful and try to sweetspot the ledge. Marth can easily punish your UP+B with Fair or Nair, Dancing Blade, or Usmash as you're coming down. Alternatively you could occasionally try to land your UP+B on the stage right at the apex of your jump which might be unexpected.
DDD's chaingrab gives him more control over the match as he can either chaingrab the Marth off the edge and attempt a gimp or go into Mindgames and try to predict if the Marth will use UP+B. One of the most dangerous places for DDD in this matchup is the ledge. Marth can cover all his ledge options with ease and it can be difficult for DDD to get back on the stage. If you're on a high percent or winning on stock you can jump -> fastfall inhale from the ledge which most Marths won't be expecting and swallowcide.
Don't always punish shield breaker as its ending lag is quite short. Unless it is fully charged it will always take away 80% of your shield so watch out. Also remember it can be used as a recovery method as he gains a lot of horizontal momentum after it. When using Bair, use a retreating Bair instead of an approaching one as it will clank with Marths Fair.
Good stages: Final Destination, Castle Siege, Halberd, Pokemon Stadium 1
Final Destination gives you good room to chaingrab and lets you slow the match down to the pace you want. The lip makes it good for stage spikes as well. Castle siege is good for walk offs on the second stage but beware of chaingrabs on the first and third stage where it is not flat. Halberd has a low ceiling so its good for killing him early. PS1 is great for DDD as he can chaingrab marth to a wall and infinite him there, though beware that Marth can chaingrab you to a wall and infinite you with Dtilt so beware of that.
Ban: Battlefield, Brinstar, Yoshi's Island, Norfair.
Marth can punish your UP+B with the platforms on these stages and can generally abuse the platforms better then you can. On Norfair his aerials>yours and your side b is useless.
