Explanation

The given sequence appears to be obtained by dividing the previous term by 3:

2187 ÷ 3 = 729

729 ÷ 3 = 243

243 ÷ 3 = 81

81 ÷ 3 = 27

27 ÷ 3 = 9

9 ÷ 3 = 3