ok so D3 has a few things he can do
ftilt- beats side b head on, stops ice blocks, trades with blizzard. you can perfect shield it into a dash grab very easily however. you can also go over it with side b or something and get him
dair- probably d3s favorite tool in this matchup (coney seemed to love it), dont try to grab until he actually lands because hes just gonna keep jumping above you like an idiot until it pokes. just count jumps and **** him for stayin up for so long. i think someone said up b beats it too but idk lol.
grab- if he catches you offguard with a grab he will bthrow you then immediately fsmash/bair nana. luckily with good reaction time you can fsmash or anything really to break the grab with the other climber.
bair- they'll really only use this offstage and its a ***** to get around, i just avoid being offstage at all times, stay towards the middle of the stage.
thats pretty much all D3 can do
now ICs have some pretty cool tools in this matchup
side b- think of it like Mks tornado vs D3 but harder to use (sorta) but it also combos. you can basically combo it into a couple uairs and if you're good you can bait em to airdodge then grab as they land.
ice blocks- ice block camping the center of the stage seems to work pretty well for me when i dont want to risk being thrown offstage ever.
blizzard- i actually rarely use this in this mu but D3 is fat and if youre smart you can land these well, just waitch out for ftilt.
nair- combo-y approach of sorts, D3 is kinda fat so i find it easy to land these on him, can combo into tight ****.
in a nutshell just use your moves that "combo" and keep D3 where you want him and combo the **** outta him. you can actually 0-70 D3 without grabs lol
EDIT: oh yeah, any D3 that inhales is bad at the Matchup, just jump at them with nair or blizzard or ice block then grab
or if theyre falling with it (like **** heads) then roll behind and grab them