How is the dominant dragon decided?

I tought it was just the last dragon, or the dragon who will spawn the most, but I did a game and got Ocean, Wind, and then on the third dragon it was decided it was a inferno map. And then 3 inferno drakes spawned in a row.

So when and how is the 'dominating' dragon decided?

Solution 1:

The first 3 dragons of the game are random and all unique, so you can never have multiples of the same elemental dragon until the map is transformed. The third dragon which spawns in a game will trigger the map changes and be the only type of elemental dragon that will spawn for the rest of the game.

https://na.leagueoflegends.com/en/news/game-updates/gameplay/preseason-2020-gameplay-rise-elements

Rise of the Elements will bring SR to life as the primal magic of the Elemental Drakes changes the landscape each game. The third drake transforms the Rift before it spawns and after that, its element will be the only one to spawn for the remainder of the game.