The pattern is that you are dividing by 3.
Therefore, to work out the next three numbers, you need to divide the last number by 3.
So you'd do:
1/9 ÷ 3 = 1/27
1/27 ÷ 3 = 1/81
1/81 ÷ 3 = 1/243
Therefore, the next three numbers are 1/27, 1/81, and 1/243.