here we go peach mainers, im a peach main and i play with 3 friends who play with a variety of characters, i'll try to outline my strategies for each one and maybe help some of you new peach mains out a bit.
1. Marth, whatever those marth boards are saying 7:3 in his favor is BS, and if your a good peach u know this, marth only has a few decent approaches, and if your using your turnips like u should be, he should be forced to approach, most marths will choose the SH double Fair approach, just wait for this one and shield grab after the 2nd Fair or the first one if he's close enuf, another approach i've seen is the B> combo, u know the one where he swings his sword in succesion and usually finishes by the quick jabs, just roll behind it when u see it coming and Fsmash him from behind. In the air marth does very well, so what i do is bait his attacks float in and out of his attack range and Fair or Bair him when he misses, and once he's off the edge, turnips til he gets close and then go for the Fair, a good combo starter is Dair as well, but its hard to land on marth because of his large hitbox on his Fsmash so use it sparingly, and remember all of your quick ground attacks, spotdodge his Fsmashes and Dsmashes and hit him with a quick Dsmash or AA combo, if played properly marth shouldn't give you too much trouble.
2. Zelda, honestly, this matchup is really going to be a defensive one, both peach and zelda play defensive so don't lose your patience and run in attacking in a furious rage, thats how zelda punishes. space with turnips, and once in a while pick a turnip but don't throw, zelda will sometimes use Naryus love thinking you will throw the turnip, and thats when u go in with a Fair, and once u manage to get zelda in the air is when u start going on the offensive, as long as u stay away from her Fair/Bair u should be fine, and zelda is rather light so getting a killing blow with a Fair or Bair or even a Fsmash shouldn't be too hard. its just alot of patient defense that wins this match. and on another note, do not go for the floating Dair, as Zeldas Usmash has insane priority over it and u will almost never land it on a good zelda.
3. Metaknight, well, my advice here is to stay out of the air as much as u can, Toad and B> work surprisingly well in this matchup, try to bait metaknight into rushing you, and try to avoid his attacks as best u can and spam Dsmash AA, and Ftilt, and once u get him in the air, stay right under him, metaknight knows that your Usmash will kill him very early so he will stay back and go on the defense, if not and he tries to come down on u with a Dair teach him that its not a good spot to be in Usmash or Utilt him and he'll learn, and as for edgegaurding, peach is someone that can actually decently edgegaurd meta, her Fair outprioritizes his so just wait for your opportunity and strike, but this matchup isnt easy, its probably one of peaches hardest matchups, but it is winnable.