Some characters are better off not being hit with dair unless you know for a fact that you can do it, which is kind of hard. For Olimar's uair, it really depends on his Pikmin. They're different each time unless they're the same color one after another and the Yellow Pikmin are ones that can't be easily killed. Gimping Olimar shouldn't be too much of a problem if you've got him at a horizontal part of the stage or below the stage. You can just edge hog him. Olimar is a tricky little ******* that shouldn't be tough for Ness, but can be simply because all his attacks look the same. His grab is by far the best long grab in the game. If he throws Pikmin at you, use your nair.
If he's trying to grab you, use PK Fire. PK Fire goes further than his grab. Just be sure you're not close by him. Nair is somewhat effective against Olimar, so try and use that, but don't rely on just that one attack to win.
Zelda is tough for Ness as it is. Oddly enough he has a better time against Zelda than he does Sheik. When Zelda is off the stage, there are a number of things Ness can do to her. He can dair her, use PK Flash or PK Thunder 2 on her if you can predict where she's going to land. If you truly want to dair your opponent off the stage, you can use fair > second jump > dair. Something similar to that could and does sometimes work.
Okay, Mr. Game & Watch is really annoying. There just isn't any point in trying to dair him off the stage. His recovery is good and on stage he can just use his uair to make it seem impossible to hit him with your dair. However, it's not impossible. It's just really difficult. It's like trying to dair Sonic, it's just not that simple. Of course, again, you can try to use an attack method that leads into Ness' dair.
I'm not sure what these three are doing that make it hard for you, so telling me might help, but I can say this about Zelda. If you use PK Fire and she gets caught in it, there is your chance of jumping into your PK Fire and using your PSI Magnet to absorb it for health. Her smash attacks are the attacks you'll have to Smash DI out of. You're better off not spot dodging Zelda, ever. Her smash attacks have that lasting effect where you do not want to shield grab her or ever let go of the shield. If she's doing this, try and roll away from her, not around her, but away from her.
That's another thing. Don't ever roll around Zelda. Her down smash is pretty powerful. Okay, on to Mr. Game & Watch, again. He's not an easy match up for me, but I'm told that the match between Ness & G&W are 50:50, which I don't actually see being true. However, here are some things I've learned fighting G&W. If he uses his Pump or Manhole on you, YOU can Bat him. Of course you have to be very close to the attack, not to the point where you're getting hurt, but you can Bat him. I fought a G&W main about a week ago, four stocks, killed him three times with my Bat and the last time with my bair.
Ness' fair is something you'll want to use against Mr. Game & Watch. G&W has ridiculous attacks. He's a character you don't want to roll around, too. His up smash is very strong and has a big hit box. His f-smash is also deadly. Don't rely on smash attacks against Mr. Game & Watch. Use more tilts or neutral A's. You can use PK Fire on him, but don't make it predictable or he'll just absorb it with his Bucket. If he grabs you, try and DI because he'll most likely use his down throw > down smash. Avoid this. But anyway, Ness' fair should do good against G&W. G&W likes to use his bair a lot, so know that he'll use this and know when to dodge it. Yes, dodge in the air. Also, abuse your grabs whenever you can. Ness' grab is pretty good in Brawl and he can back throw his opponents.