Okay, about Little Mac.
I really don't like Luigi's chances against Mac. However, your saving grace is that Mac players online are stupid (good luck against Mael, Godot or myself though : P) and you can often play the waiting game, keeping him at bay with fireballs and what have you, until he makes a mistake.
Practice shield grabs. The idea is to wait for Mac to come to you and when he gets close he'll probably throw out a dash attack or side B. Shield easily stuffs both of these, and then you can grab him and do Luigi things to him.
Don't expect to get an opportunity to get a solid up B "shoryuken" unless the Mac player spams rolling dodges. In fact, I would not use any of Luigi's specials offensively outside of his Fireballs; using the Side B, Up B and Down B is very risky due to their end lag which allows Mac to just eat you alive.
Mac's recovery is laughable, and is easily gimped when he needs to recover horizontally. Thus at high percentages look for attacks which will send him flying horizontally rather than vertically.
Godot has way more experience with that matchup
The Link-Mac matchup is imo laughable, especially after the huge buff to his grab game - when you get enough practice with the matchup.
Like with Luigi, shield grab is a viable strategy in Link vs Mac. Except you launch him way more easily than Luigi does, and you can gimp him with bombs and boomerangs and such when he's offstage, or you can be a man and Down-air him or something.
Don't get too reliant on projectiles and long range grabs, as Mac can (and will) find a way around him
Again, Godot is the best person to ask here.