a/s/l & facepic plz
Doc, Luigi, Mario like to crouch cancel into down smash a la Peach & Samoose because they think they're so clever, preying on weak moves like f-tilt & jab.
Against Luigi:
Use a lot of needles (aerial & ground, depends on how far he is), fair & bair, and d-throw > f-tilt > block at low percent (after like 30% switch to fair or uair [if he doesn't DI away just fair > ??? or uair always]). Run off platform fair. Shield > punish (sometimes WD oos is required) is good against his entire moveset except grab & super spaced f-tilt & some down smashes he'll get enough momentum to be safe from you on (although WD oos needles actually can work on the latter & is ****ing annoying).
Use needles to stop misfires preemptively during his recovery and to prevent him from getting distance from tornado & green missile. Fair > bair if you can hit it on him. Take away all his options & hog him (or hit him away).
You should DI all his throws behind him. CC to block, d-smash, or grab if he does jab stuff.
Doc:
SH fair. SH bair. Run off platform fair. Use a lot of fair & bair. Space them. Air needles are pretty good too. Grabbing him is really good, too, and your ground movement is better. F-tilt is dangerous against shields if they're good but most of them suck (they can up smash oos but only Shroomed does it). Crouch cancel > block, d-smash, or grab if he jabs (Docs like to jab > d-smash so CC > block > WD oos punish works a lot). His game is mostly centered around crouch-punishes, grab attempts, and getting under you with up tilt, uair, or up smash so if you play to avoid those you'll be fine. Your f-tilt ***** his aerial approaches.
If he camps pills, just charge needles and then prepare to engage him. It's easy to run under them, WD under them, crouch them, nair through them, bair to protect yourself from them, etc.
The combo tree is similar to that of Marth. On DI away, d-throw > dash attack isn't 100% guaranteed (jumpable, possibly even vulnerable to Up+B) but it works enough that I recommend it anyway up to about 45%. After that, f-tilt or fair (or get creative with nair or something).
Mario:
Treat it exactly the same as Doc only expect Marios to be more aggressive about holding the middle & stuff. If you go berserk with aerials you might get f-smashed so play a bit more grounded. You can also be f-smashed if you go for nooby shield grabs and whatever on spaced moves, so don't do that. They're gonna try to bait you into being f-smashed a lot. So block, stay spaced, etc. It's not a great plan on their part or anything, but it's easy to get complacent when you have a big character advantage. If you deny them their random smash attacks & grabs, there's not much Mario can do about you.