How to get a average for part of column data by using formula

[#1]

For example, I have a column:

2

3

4

50

60

70

would like to get the average for every 3 rows, and put the average in a new column in first row in every set by using formua, like this: 

2  3

3

4

50  60

60

70

[#2]

@jasongao, 
     One way would be something like this: 

     You will need to extract your data from column into a matrix. You can do that using << get as matrix command. 

Mat = [2,3,4,50,60,70] ; 

NewMat = Transpose(Shape(Mat,(N Rows(Mat) * N Cols(Mat))/3,3 )); 

V Mean((NewMat)); 

[#3 · Accepted solution]

Try this:

If(Modulo(Row(), 3) == 1,
    Col Mean(:Column 1, Ceiling(Row() / 3))
)

[#4]

Thanks! It works!