First off, Nair is two hits.
Secondly, I approach D3. Zone with Nair and jump back Fairs until you find a break in his defense. You can run in and shield and punish a Bair, or powershield and punish an ftilt (takes practice) Waddles are extremely unsafe. I also cross-up a lot. Once you get D3 offstage, keep him there.
You should edgeguard MK from the edge. You can counter drill rush, and he can't use DownB if you're holding the edge (just ledge hop Nair through him to punish and hit him back out). In my older threads, I have extensive guides that still hold true on how to edgeguard MK, so I suggest you find them. Marth beats MK in the air, so you want to capitalize on the edgeguard as much as possible. From the edge, dropping leaves your back towards MK, and your Bair wrecks him, so it's all good. Counter if he uses a priority ball attack like Drill Rush or Nado. From the edge, jumping into counter or UpB is tricky, and you kinda have to predict it, but it will generally be the third option MKs choose.
I was under the impression that we cannot guaranteed GR Dair MK. Did I invent this tidbit in my head?
Nair is good because it is a linker, and it protects Marth for a longer period of time. It also has incredible knockback on tipper, first hit autocanceled into the ground has great frame advantage, and it shield stabs larger, harder to kill characters very well. It is pivotal and cruicial for zoning in some MUs, including R.O.B., D3, and DK, who are all difficult MUs for Marth.
I personally fight Snake on Neutrals that I am comfortable on. You want to rely on performing solidly, and air and edge-pressure. Brinstar isn't bad though, and neither is Rainbow Cruise.
If Kirby is hitting you with Bairs, it means you are spacing poorly, or getting baited. Marth outranges and outspeeds Kirby, so just wait patiently. If you swing first and miss, Kirby is free to hit you with Bair. Instead, wait until Kirby swings first. Shield if necessary, then jump and Fair him, or punish in the appropriate manner. Simply walking on the ground should make it easy to avoid Kirby's Bairs. I'm also pretty sure jabbing and dashing into shield is effective.