I'll talk about the Peach/Marth matchupHi, can I get some general tips as to how Marth is supposed to fight Peach and Sheik? Thanks
Peach wants to be in one of two places: 1) Far away from Marth so she can pull out a turnip 2) Inside of Marth so she can start to punish Marth.
Peach wins far away and close range. Marth wants to stay in mid-range distance from Peach. Close enough so she won't pull out a turnip and far enough so Marth's range beats anything Peach can do.
A good Peach will stay grounded in the matchup. When Peach is on the ground, use a lot of a dash dancing. Force her to commit to something. If Peach sees Marth DDing, she will probably dash attack at some point. Be aware of that. Try to get grabs, dash attacks, and d-tilt when she's grounded.
If she goes up in the air, just fair her. Peach will not be approaching you from the air though. Peach will generally be in the air when she's trying to run away from you (to get a turnip) and doing retreating bairs. Her bair protects her from a grounded Marth approaching with d-tilt/dash attack/grab. If Peach catches on to you fairing her bair, she will start to dash attack again to beat your fair.
I think the matchup is close to even but slightly in Marth's favor. The matchup relies heavily on being able to get a feel for your opponent and understanding their habits.
Some good stages to pick: Yoshi's Story, FD, and Pokemon Stadium
Yoshi's doesn't give Peach room to run, FD is just a great Marth stage and a grab heavy stage, and Stadium has good platforms for Marth and a low ceiling to kill Peach. I'd say the hardest part of the matchup is killing Peach. Start looking for your kills early, not when Peach is racking up damage to 120%+ because then you're in trouble.