A Grand Trine is an aspect pattern, where 3 planets are in a triangle shape, each 120° apart. TThis figure shows free-flowing, cooperative interaction and natural ability with the planets involved. The aspect figure takes the shape of an equilateral triangle.
The planets are typically in signs that share the same element: Fire, Earth, Air, Water. Generally, this is an easy-flowing energy, indicating natural talent particularly in the element involved. However, this natural ability may come so easily that it is taken for granted and under-used.
In Fire signs (Aries, Leo, Sagittarius) this denotes enormous courage, enthusiasm and action...although this can also take the form of risk-taking, impulsive actions and a short-lived attention span. Discipline and substance are needed to sustain the energy.
In Air signs (Gemini, Libra, Aquarius) this may give intelligence and communication ability, and a love of gathering and exchanging information...as well as perhaps being excessively analytical or intellectual. Their thoughts (and communication) may flow so freely that it's hard to harness them enough to put them into concrete form.
In Water signs (Cancer, Scorpio, Pisces) this indicates someone who is deeply feeling, sensitive, intuitive and creative...although they could also be overly vulnerable, moody or have difficulty with analytical or logical tasks.
In Huber astrology, this is called a Large Talent Triangle.