Triangle Attack

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: Ankoku Ryū to Hikari no Tsurugi for the pegasus sisters. The participant who then attacks in that position will land an automatic critical hit. In most games, only the three Pegasus Knights/Falcon Knights are able to carry it out, 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.

The Trios

Ankoku Ryū to Hikari no Tsurugi

Palla, Catria, and Est.

Gaiden

Palla, Catria, and Est.

Mystery of the Emblem

Palla, Catria, and Est.

Genealogy of the Holy War

Vampa, Fetra and Eliu. (All three are variants of the Mage class.)
Meng, Maybell, and Bleg. (All three are Falcon Knights.)

Genealogy of the Holy War features the only two trios that are actually enemies rather than allies.

Binding Blade

Shanna, Thite, and Juno.
Bors, Wendy, and Bath. (All three are Knights.)

Rekka no Ken

Florina, Fiora, and Farina.

The Sacred Stones

Vanessa, Tana, and Syrene. (Only one may be a Wyvern Knight, and if so, that unit has to initiate the triangle attack.)

Path of Radiance

Boyd, Oscar, and Rolf. (The player must have read the Info Conversation between the "3 Brothers" (Chapters 13 and 19). In order for the second one to appear, Oscar and Boyd must both be promoted and able to wield bows as their secondary weapons.)
Marcia, Tanith, and Elincia. (The player must have read the Info conversation "Holy Guard" (Chapter 28). Marcia must also be promoted by then in order for it to appear.)

Radiant Dawn

Marcia, Tanith, Sigrun, and Elincia. (Note that only three of them may take part, hence it's a Triangle Attack.)
Boyd, Oscar and Rolf.

Shadow Dragon

Palla, Catria, and Est.

Shin Monshō no Nazo ~Hikari to Kage no Eiyū~

Palla, Catria, and Est.
Barst, Cord, and Bord.

Since Shadow Dragon has introduced the Reclassing system, some players wonder if the three sisters are still qualified to perform a Triangle Attack. The fact is that they are, 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.

Battle Cries

Vampa's Quote Initiating a Triangle Attack — Vampa preparing to attack Teeny in *Fire Emblem: Genealogy of the Holy War*.

Usually, a character will have something to say the first time he or she triggers a Triangle Attack.

“Fetra, Eliu, let's go, right now! TRIANGLE ATTACK!!”
—Vampa's battle cry in Seisen no Keifu

“Juno, Thite, let's go! Triangle Attack!”
—Shanna's battle cry in Binding Blade

“Juno, Shanna, get ready! Triangle Attack!!”
—Thite's battle cry in Binding Blade

“Thite, Shanna, get ready. TRIANGLE ATTACK!!”
—Juno's battle cry in Binding Blade

“Uh... let's go. TRIANGLE ATTACK!!”
—Wendy's battle cry in Binding Blade

“Show the enemy the power of us Armor Knights. TRIANGLE ATTACK!!”
—Bath's battle cry in Binding Blade

“Get ready! Charge! Triangle Attack!!”
—Bors' battle cry in Binding Blade

“Um... Here goes... Tri... Triangle attack!”
—Florina's battle cry in Rekka no Ken

“Farina! Florina! Remember our training? Triangle attack!”
—Fiora's battle cry in Rekka no Ken

“Behold the dancing wings of Ilia! Triangle attack!!!”
—Farina's battle cry in Rekka no Ken

“I know we can do this! Triangle attack!”
—Tana's battle cry in The Sacred Stones

“Here I come, Princess Tana, Syrene! Triangle attack!”
—Vanessa's battle cry in The Sacred Stones

“Behold, the secret strike of Frelia's pegasus knights! Triangle attack!”
—Syrene's battle cry in The Sacred Stones

“The time is now! Triangle attack!”
—Rolf's battle cry in Path of Radiance

“Now!!! Triangle attack!”
—Rolf's battle cry in Radiant Dawn

“Look, an opening! Triangle attack!”
—Boyd's battle cry in Radiant Dawn

“Now's our chance! Ready... Triangle attack!!!”
—Oscar's battle cry in Radiant Dawn

“Now! Now's our chance!!! Triangle attack!”
—Marcia's battle cry in Radiant Dawn

“All together, now! Triangle attack!”
—Tanith's battle cry in Radiant Dawn

“So, are we ready?! Triangle attack!”
—Sigrun's battle cry in Radiant Dawn

“...Let's go! Triangle attack!”
—Elincia's battle cry in Radiant Dawn

“With me, sisters! Triangle Attack!!!”
—Palla's battle cry in Shadow Dragon

“We have you now! Triangle Attack!!!”
—Catria's battle cry in Shadow Dragon

“It's just you and me! ...And my sisters! Triangle Attack!!!”
—Est's battle cry in Shadow Dragon

“Bord, Cord, you's ready? Triangle Attack!!!”
—Barst's battle cry in New Mystery of the Emblem

“Let's do this, bros! Triangle Attack!!!”
—Cord's battle cry in New Mystery of the Emblem

“You're no match for the three o' us! Triangle Attack!!”
—Bord's battle cry in New Mystery of the Emblem

Formation

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

 U
UEU

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.