I'm leaning towards the first one. If you want to test it I had RNG'd each bird one at a time then purposely lost the battle once the bird I RNG'd for was sent out (Moltres first, then Articuno, then Zapdos). It is possible to RNG 5 of the 6 shadow pokemon this way (you have to pick between Moltres and Rhydon).
So when I got to Zapdos it was the only pokemon left not already determined. If there are more pokemon to be determined you should be able to hit the spread if I was stopped by opponent's ID/SID but that means you would miss out on Articuno.