How to calculate recent row average value using MDX?

My data example:

id  value_a 
1   1.0
2   2.0
3   3.0
4   4.0

which I want is

id / value_a / recent_n_avg

1    1.0     0.33

2    2.0     1.00

3    3.0     2.00

4    4.0     3.00

recent_n_avg is average for recent n rows( n=3 in example).

How to use MDX to solve this problem.

Thanks.

If you want do it only for one dimension and this dimension is flat you can write following expression for new calculated member:

SUM({[Dimension].CurrentMember.Lag(2):[Dimension].CurrentMember}, [Measures].[Your measure])/3

Also, you should remember about member "All" and members # 1,2 in your dimension.