Gemini, Libra, Aquarius. The signs of relation — comparing, connecting, standing apart to see.
Air signs sit at 60°, 180° and 300°. Like the other elements they are mutually trine, and like the others they contain one sign of each modality.
Aquarius is an air sign despite the water-bearer in its name and symbol, which is probably the single most common confusion in the whole system. The jar is being poured, not swum in.
Libra sits at the September equinox, the exact midpoint of the circle at 180° from Aries — the only element that contains one of the two equinox points and one solstice-adjacent sign at once.