Against Marth I like to lose really fast so I can pick Shiek, against Roy I like to win really fast so he can pick Peach.
Seriously though, Roy is garbage you can CC his aerials into grabs and he is much laggier and has to play a more risky closer game vs ICs, against a solid Marth with good spacing its very hard to land a hit, I notice that I use a lot more rolls in this match up and I probably shouldnt, if you manage to land a grab do your best to put him in the air so you have a priority advantage, level with and above Marth he has a lot more reach than you and thus a larger priority, when you are below him he doesn't have many options, I like to dash dance a lot and if the Marth messes up his spacing or timing you can go for a grab, also you can shield plenty against Marth just wait for oportunities to have the other IC shield grab him if he grabs, you can't really go towards Marth safely, but he can't really approach you safely either, when either is still the other cant effectively approah, if the Marth DIs towards you with Fairs you can dash dance grab. If he is stationary or moves away you can't do anything about it. If you block a DIed towards you fair he has two choice double jump or fair again, if he fairs again you should have time to grab him, if he double jumps you can short hop a uair unless he jumps away from you, if he does that you can gain some ground but it isn't really safe to go within his sword reach unless he is in lag.