.
The three terms are a, ar and ar^2.
Their product is a*ar*ar^2 = (ar)^3 = 216;
hence, ar =
= 6.
Thus the middle term of the progression is ar = 6.
3 seconds to guess two other terms: 1 and 36.
So, the progression is 1, 6, 36.
The reversed order 36, 6, 1 also works.