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.