Etydhv

Viserion is felled by one spear. Rhaegal is felled by 3 arrows. Drogon cannot be hit once, even with dozens of scorpions aimed at him.

Why are the other dragons so easy to hit with projectiles but Drogon isn't?

Is it plot armor, or is there another reason?

Drogon is the biggest

Drogon is essentially the big brother of the three. He's the largest, potentially the fastest and most probably the strongest; he's the alpha.

Drogon has Dany in the pilot seat

It would make a lot of difference that Dany is the one riding Drogon, able to give commands, and steer him better than the other two may.

Drogon has the most combat experience

Being Dany's main ride, Drogon has experienced a lot of attacks, he has gained the most experience under attack than his two siblings.

Viserion was hunted by the Night King

Olympic level javelin throwing is an understatement for this guy. The Night King took down Viserion as they were retreating with an epic throw of his spear. He's powerful, that spear was travelling very fast, and hard. It pierced Viserion exactly where it needed to to mortally wound him.

Rhaegal was injured already

In the scene where Rhaegal is shot down by Euron's scorpion, we can see that Drogon and Rhaegal are flying in a tight, low and slow formation. Rhaegal is visibly injured, his wings torn from the Battle of the Long Night. This makes him slower, and a much easier target.

Besides, Euron had the element of surprise; he could take his time with his shots and make it count. Notice how once alerted, Drogon was able to evade the rest of the arrows?

Drogon was in fact hit once

Recall in the Seventh Season, Drogon is hit by Bronn's scorpion, severely damaging his shoulder. So he's not invincible.

Viserion is a different case because he’s killed by the Night King and an ice spear so we can rule him out from your analysis as the case isn’t the same.

You also make a slight mistake in that Drogon does get hit by a scorpion bolt but that was back in Season 7 Episode 3, “The Spoils of War”, on the first design by Bronn during it.

Rhaegal is killed by poor writing essentially but you can explain it by that ”she just kind of forgot about the Iron Fleet” and the fleet having all the time in the world to line the shots up. However, even then it is next to impossible but hey ho.

Drogon in that scene then “dodges” the bolts coming at him.

In the next episode, “The Bells”, Dany and Drogon use the scorpions weaknesses against them; they are slow turning and take a while to load and are quite hard to aim. Dany moves around a lot and only comes in close after a volley has been shot and when close moves around even more whilst minimising the targets.

What we’re really comparing here is the difference in writing from one episode to the next. The killing of Rhaegal is poor writing and portrays the scorpions as extremely over powered yet at King’s Landing they are shown to be pretty realistic. That is the difference not necessarily that Drogon is better. Although note Drogon also has Dany on him to help command him around the battle.

And it’s also worth noting as I hinted to above that Drogon does not have dozens of scorpions pointed at him because they can’t keep up with the dragon to aim properly.

Apparently, the element of surprise is a Dragon's real Achilles Heel. The Night king got his dragon when nothing else was attacking those dragons, so Viscerion gets hit out of nowhere by an Ice Lance. Rhaegal bit the dust as they were on the way to the capitol and were ambushed(though more of a plot ambush, who isn't expecting some kind of attack???!)
In both cases, they didn't even know the first attack was coming. Were they weaker than Drogon? I can't say-but he never faced an attack without knowing where it was coming from.