The Triangle Attack is a powerful move that can be performed by certain trios of characters should they surround an enemy from three sides before attacking. This special attack was introduced in Fire Emblem: Shadow Dragon and the Blade of Light for the pegasus sisters. The participant who then attacks in that position will land an automatic critical hit. In most games, the Triangle Attack is carried out by three Pegasus Knights/Falcon Knights, however, there are a few exceptions.
Triangle attacks are generally considered impractical on normal enemies, both due to the relative difficulty of setting them up, and due to the fact they usually require you to raise three units of the same class, while variety is generally more desirable. Triangle attacks are suggested to be saved for difficult bosses.
Even with the Reclassing system introduced since Shadow Dragon, the three sisters are still qualified to perform a Triangle Attack, no matter what class each of them is assigned, as long as they are all adjacent to their target.
It is also worth noting that since the Triangle Attack is not considered to be a critical hit, it can bypass the effect of the "Dazzle" card in WiFi battles.
The attack can be performed by any of the three participants, from any side, as long as all three of them are surrounding the target enemy. The formation of a Triangle Attack usually is as follows:
U = Allied Unit
E = Enemy
In the case of the three brothers, Boyd, Oscar and Rolf from Path of Radiance, bows are used instead of short-range weapons, therefore each of the participants needs to be one square away from the target.