What are the Left Flank, Center and Right Flank positions in an army?

An army fights with 3 dragons placed in 3 positions: Left Flank, Center, Right Flank. Only those 3 dragons fight.

The position that matters most is the Center: it's the hub that spreads auras across the whole flank. An aura is a team bonus (e.g. cutting enemy damage by −27.5%, or boosting everyone's Initiative).

So the basic rule:

1. Put the dragon whose team effect gains the most from being spread at the Center — usually a Champion.
2. Put your big hitters / damage dragons on the flanks.

You don't steer the order of attacks during the fight (Initiative decides that). So you "play" beforehand, by choosing which 3 dragons you bring together and where you place them.