Okay, some quick advice:
Kirby vs Fox vid
Don't get u-air'd. That's always good advice. Ha ha. When Fox is at higher percents, don't go for F-airs (unless they're the only thing that can hit). B-airs and U-airs can get him off the stage, or outright KO him. When you used a ledge-cancelled Final Cutter at the end, you should have ledge-hopped into a Swallow and caught him as he shielded. Kirbycide FTW! Instead your F-air was blocked and you got u-aired.
Kirby vs Peach vid
If you're going to get grabbed by Peach, at least DI behind her, no sense in getting up-smashed if you don't have to. You can crouch under her grab attempts (just remember to avoid her D-smash). F-airs are pretty good approaches on her, as they can get her into the air, especially if she happens to DI up. Against crouch-cancelling Peaches, use D-air (and grabs/Swallow). She's more vulnerable when above you, so hit her with U-air. U-tilt to B-air/U-air is too good. U-throw at low percents, D-throw at high percents.
Kirby vs Falco
The best thing you can do in this situation is learn avoid and DI Falco's attacks, because, let's face it, Falco is faster, stronger, and combos more easily than Kirby will ever be able to do. Crouch under his laser spam (Power Shield as they fly over your head to send them back at him). He will usually go offensive after this and move in for an attack, generally N-airs and D-airs. If you're doing your job of avoiding his attacks, you should be able to get in some attacks of your own; make sure you're working at getting him off the stage as soon as possible (B-air, F-tilt, D-tilt, B-throw). From there, edge guard (D-tilt, D-Smash, B-air) if he can get back onto the stage, or if he's out a bit further, grab that ledge and edge hog him (you can ledge-drop into B-air/U-air to keep him away). U-tilt and D-throw when you can get them in for juggles and tech-chases.