PoppaSquat
Before I get onto my nitpicky things, there's one thing that I want to focus on: last stock of game 3. You were at 0% and he was past 100%, meaning that killing him should have been relatively easy. So that got to your head and you threw out fairs, fsmashes, and nairs willy-nilly. You're honestly just lucky that you didn't get punished for this too much. Even if your opponent magically lives to 350%, don't let it get to your head. Instead of thinking that they're one hit away from dying, take note of how they treat the fact themselves. Do they try to camp you so they can get one clean hit to finish you off? Do they play super aggressively and try to overwhelm you with their sheer number of hitboxes and get earlier KOs? Do they opt to play super safe and avoid trades, instead waiting for you to overcommit? Think about all of these things when you get your opponent around 130%, because a solid shine or usmash OoS could've been super bad for you that last stock.
Nitpicky things:
You had a lot of good ideas, such as fairing the startup of Firefox or punishing his approaches with fsmashes. The issue was timing. There were one or two times I saw you go to fair Firefox, but you moved too late and got hit for it. Don't let that happen.
Jumping out of hitstun. I saw you do this after Fox launched you upwards during a trade. I'm relatively sure you could've wiggled the stick, wavelanded forwards, and gotten your positional pressure right back on, but instead of keeping it personal, you gave him back a nice portion of the stage to work with. I think this was game two. I know that you won, but it's still a relatively important mistake.
Edgeguarding. Against a recovering Fox, I personally think that using a WD edgehog to cover the sweetspot should be the first thing you do. Granted I main Falco now and can cover a lot of angles with my jump and bair, but Marth can still cover the non-sweetspotted angles just fine. Ledgehop uair covers a lot of them in and of itself, and always remember that you don't have to have somebody in hitstun to be beating them. If they land on a side platform, but they can't shield drop and you can cover the whole platform at once with a SH delayed uair, go for it. Use positional pressure and intelligent spacing to keep them on edge. An antsy opponent is far more likely to screw up and give you a crucial opening.
Adaptation. Granted that your opponent liked to mix up his stuff a bit, but you still had room to adapt. I honestly didn't really see you just try to bait him out or anything like that too much. It felt like you wanted to be in his face, and his defensive game was why you got outplayed sometimes. I know this might be hard to visualize, but try to get in that ONE area that he doesn't want you in. Maybe it's just a good dash away, maybe it's super far from him. There's some position that he won't like. If he wants to SHDL when you get far enough away, stay close enough that you can punish SHDL on reaction with a grab or something of the sort. Get creative. The simplest example of adaptation I can think of is the only tourney set I've won to date: I saw that the Fox I was going against wanted to drillshine usmash me whenever I got close enough, so I just baited him out by going either under it or WDing back at the last second. Your movement is definitely good enough for this, just work on reacting accordingly.