I`ve only been playing fox for a couple months, but from what I hear and my own experience, all three of those matchups are quite fox favored.
Peach and Puff are pretty slow, so as long as you don`t play too aggressively you can outrun them in the neutral, so SHL (or SHDL if you can consistently) and get them to approach you and they`ll be at a decent disadvantage.
Sheik`s main weapon against you is her dthrow tech-chase, which can seem horrible if you start to panic. Make sure you mix up your DI and tech rolls, try to go for tech in place shine a little more often than the others because it`s the hardest to react to.
Fox is good. Like he`s absurdly good. Developing a single character well is almost always be a better option than picking up a secondary (especially when your character only has favourable or even matchups). Just keep practicing.