Some basic tips:
ROB: probably utilize side B some (not just to reflect projectiles per se), figure out how to time your nairs to beat pellets and other MM aerials, remember your laser can go through Mega Man's projectiles. ROB can get some combos on MM so part of it will be taking advantage of when you get in.
Samus: play campy and charge your charge shot, it beats every projectile Mega Man has. Get good at charging in the air, cancelling your charge, etc. so you can keep it charged as much as possible. Be aware though that if pellets hit you during the startup of charge shot, you'll lose it. But in general just having the charge will make MM have to play very differently since it's a very bad trade to toss out a Metal Blade and eat a Charge Shot in exchange so they'll have to play around it a lot. Utilize bombs to mix up your landings and potentially eat some of MM's projectiles.
I don't feel like MM has an amazing matchup vs. those characters, so you just need to learn how to get around those pellets.
For Ness, a good MM should be very careful with f-smash due to the bat and PSI Magnet, and remember Ness can absorb the explosion from Crash Bomber. But since Ness doesn't absorb Metal Blade, Leaf Shield, Hard Knuckle or Air Shooter, don't overuse it. I love a Ness that constantly tries to anticipate an f-smash because that means they'll eat a lot of Metal Blades, Leaf Shields and aerials.
