The standard deviation is the Square root of the average square distance between the data and its mean.
The simplest way to do this would be to choose 6 data points, and give them all the same square distance from the mean. We want an average square distance of 3^2 =9, so I will make all my data have a distance of 3 from the mean.
You might find it easier to think along these lines instead
"If the standard deviation is 3 then the variance must be 9"
"The variance is the average squared deviation from the mean. I can get a variance of 9 by putting every point 3 units away from the mean"
Either way, The easiest way to to this is to choose a mean value (ill use 4). Then pick 3 points with a positive deviation of 3, and 3 points with a negative deviation of 3.
So i suggest
You can check that the mean of the above values is 4 and the standard deviation is 3