We don't do MU discussions anymore because our boards are really lazy.
Marth generally out ranges you, so you have to be careful with your spacing. Tornado is alright vs Marth but he can up b out of it, and counter it. I think dancing blade breaks it as well, I don't remember.
MK's Dtilt and the last hit of ftilt out range Marth mostly.
If they're just spamming fairs then you can power shield one of them and grab him cus Marth is not really supposed to play aggressively vs MK. If they're playing aggressive vs you, you can pretty much read and punish everything they do