Good point, it's annoying to catch all of them. Believe me, it's EXTREMELY frustrating. But I would say that they are about the same.
Zapdos base stats:
HP
90
Atk
125
Def
90
Sp. Atk
85
Sp. Def
90
Speed
100
Total Base Stats
580 (#)
Zapdos is a good physical sweeper. It would do well in a hyper offense team with Spectrier/Calyrex Ghost Rider Form in doubles. And its ability Defiant raises its Attack sharply when its stats are lowered. It is also a good counter for itself as a lot of people will probably use it in the Tundra Tourney competition.
Moltres base stats:
HP
90
Atk
85
Def
90
Sp. Atk
100
Sp. Def
125
Speed
90
Total Base Stats
580
Moltres would be a good special sweeper, and it's ability Berserk raises its Sp.Atk by one stage when its HP drops to half or less. Run Nasty Plot with it and you'll be unstoppable. It would also work well with Spectrier/Calyrex Ghost Rider Form in a hyper offense team in the Tundra Tourney competition.
Articuno base stats:
HP
90
Atk
85
Def
85
Sp. Atk
125
Sp. Def
100
Speed
95
Total Base Stats
580
As you can see, Articuno's base stats are about the same as Moltres, only Moltres has higher Speed and Sp.Def. It would go well with Spectrier/Calyrex Ghost Rider Form or Galarian Zapdos in a hyper offense team for the Tundra Tourney. Not only that, Articuno can counter Bug types, one of the Psychic type weaknesses because of it being a flying type as well.
So, when you look at it, in terms of Special Sweeping, Moltres would be a good choice. But out of all of them, I would go with Galarian Zapdos. Its ability raises its Attack more than the others' Sp.Atk. And Zapdos is a very unique Pokemon either way. By the way, the reason I am talking about the Tundra Tourney competition is because the Battle Of Legends competition has ended. I think you are thinking of the Tundra Tourney anyway. The sign up period starts on Oct 29th 2020 and ends on Nov 13th 2020. The competition lasts from Nov 13th 2020 until Nov 15th 2020.
Sorry for the long explanation, I thought I needed to go into detail, which is unlike me!
Hope I helped!
Sources: How to catch Galarian Zapdos
How to catch Galarian Moltres
How to catch Galarian Articuno
Edit: Articuno's ability competitive raises Sp.Atk sharply when it's stats are lowered. So in theory it doesn't matter which bird you go for. It just depends what you need on your team.
