Why does my army only have one troop type in Game of Thrones: Dragonfire?

It's by design: the troop type is chosen per army, with a single selector. All 3 dragons carry the same type (Cavalry, Shields, Archers or Spearmen).

The deeper reason is that the troop type you field triggers your research percentage for that type, which is the biggest damage multiplier in the game. Since that's one single layer per army, it forces the rule: 1 army = 1 type.

So the right reflex when you build one:

1. Pick the type you've researched most, the one that fits the largest number of your dragons (green affinity).
2. Field the 3 dragons that are green for that type.

Don't mix: a dragon with a negative affinity for that type loses stats. An army is a single-type team, not an assortment.