The triangle attack is used by three units of the same class. To use the triangle attack you must align the 3 units
to form a triangle around the enemy.
To activate the Triangle attack once in formation, use a unit to attack normally, as soon as the battle starts the triangle attack will begin, giving you can instant critical hit.
Triangle attack positions example from Sacred Stones
Learning the "Three Brothers" Triangle attack
While at the base before starting Chapter 13, goto the Info menu and view the "3 brothers"
conversations between, Boyd, Oscar, and Rolf.
Promote all 3 characters and give Oscars the ability to use bows.
At the Chapter 19 Base view the "3 brothers" conversation again.
Now you will be able to use the triangle attack.
To activate the attack, you must surround the enemy by placing Boyd, Oscar and Rolf two squares away from the enemy.(So that they are in range to attack with a bow)
Attack with a bow to activate the triangle attack.
Learning the Falcon Knight Triangle attack
Promote Marcia into a falcon Knight
View the Information at the base before chapter 28
You will now be able to use teh Falcon triangle attack in the final chapter.