Trinity of magic
The trinity of magic is an element of strategy created for the Fire Emblem series that is the magical equivalent of the Weapon Triangle. The trinity of magic is absent in recent entries in the series.
The trinity of magic was first implemented in Fire Emblem: Genealogy of the Holy War when the magical classes became far more expanded. The three types of magic in the triangle were Fire, Thunder, and Wind. Fire beat wind, wind beat thunder, and thunder beat fire. Two other types of magic existed as well, Dark and Light, which both had an advantage over Fire, Thunder, and Wind, but were neutral to each other. This system was also used in Fire Emblem: Thracia 776.
In the Game Boy Advance games, the three types of nature, or "anima" magic (i.e., the combination of Fire, Thunder, and wind) were collapsed into one, so only three types of magic remained: anima, light, and dark. This time, anima beat light, light beat dark, and dark beat anima. Fire Emblem: Path of Radiance changed things up again, re-dividing the anima magics into Fire, Thunder, and Wind, but removing dark; light and anima were therefore made neutral to each other.
However, in Fire Emblem: Radiant Dawn, things were complicated even further with the re-inclusion of dark magic. The original trinity of magic of Fire-Thunder-Wind (now called the Anima Trinity of Magic) still existed, but the Game Boy Advance Trinity of Anima-Light-Dark (called simply the Trinity of Magic) was also present, over-arching the Anima Trinity.
There is no trinity of magic in Fire Emblem: Shadow Dragon nor Fire Emblem: New Mystery of the Emblem, as there were no different classes of magic in the games of which they are remakes. The trinity of magic also does not feature in Fire Emblem Awakening or Fire Emblem Fates despite those games featuring tomes of all three types of anima magic.
In Fire Emblem Heroes, the anima trinity of magic is mixed in with the weapon triangle.
Like the weapon triangle, the trinity of magic does not necessarily determine the result of a battle. If a unit is significantly more skilled than another, then the more skilled unit will have little trouble defeating the less skilled one, even working against the trinity of magic.
In addition, trinity of magic effects are determined by the magic tomes that the battling magic users have equipped, not necessarily what magic the units can use. For example, in Fire Emblem: The Sacred Stones, if a Sage (which can use both light and anima magic) has a light tome equipped and battles another sage who has an anima tome equipped, the first sage will have a disadvantage due to the trinity of magic.
Trinity of Magic Bonuses
Fire Emblem: Genealogy of the Holy War: ±20 Accuracy
Fire Emblem: Thracia 776: ±5 Accuracy
Fire Emblem: The Binding Blade: ±1 Weapon Might, ±10 Accuracy
Fire Emblem: The Blazing Blade: ±1 Weapon Might, ±15 Accuracy
Fire Emblem: The Sacred Stones: ±1 Weapon Might, ±15 Accuracy
Fire Emblem: Path of Radiance: ±1 Weapon Might, ±10 Accuracy
Fire Emblem: Radiant Dawn: ±1 Weapon Might, ±10 Accuracy
Note that the plus-minus sign (±) means to add that amount of points if the attacker has a Trinity of Magic advantage and to subtract that amount of points if the attacker has a disadvantage.