If you're looking for a replacement for dair on stage, I guess you're trying to attack someone while you're above them. That's not a good idea, because marth is pretty bad at fighting when he's above someone. Your best bet would be to get back onto the ground via an air dodge and continuing the fight from there. Don't try and fight while you're directly above someone, marth's options are limited there and you're probably just going to get juggled if you focus on reaching the ground.
In terms of using nair and bair: nair is pretty useful if you want to hit someone in a plat above you, since the two hits puts more pressure on them, and nair is relatively easy to hit with. You can also use it to juggle in general, because if you predict an air dodge and nair, the knockback will allow you to continue juggling or potentially hit them offstage for edgeguarding. Sometimes when you wall with sh fairs, you can throw in a nair as you're coming down so only the first hit comes out. The first hit will autocancel, and you can follow up with db or jab or something. I find that if I hit with the first hit of nair, I can always hit with 1 jab and tip with the 2nd.
Bair is more situational, since it's more laggy than fair, you're only really going to want to be using it for KO purposes. At best, if you happen to be facing/dashing away from your opponent and want to nail them with an attack w/o turning around for a fair. This might be the case if you're facing away from your opponent and they're rushing towards you for an attack, a bair will probably stop them. It's also better to use the bair on an opponent who's stuck on a platform above you, not someone who's on the same level ground as you.