Does each dragon have its own troop type? (a common mistake)

No, and it's a very widespread confusion. The troop type is chosen per army, not per dragon.

When you build an army you pick ONE type (Cavalry, Shieldbearers, Archers, Spearmen or Siege) and all 3 dragons carry it. You can't give cavalry to one dragon and archers to another in the same army.

What that means in practice:

1. An army is built around a single troop type.
2. You choose that type, then check which dragons are green (positive affinity) for it.
3. Ideally, you take the type that is both your most researched AND the one many of your dragons are green for.

If you believed the opposite at first, no panic — even some guides got this wrong. Just remember: 1 army = 1 troop type.