3 Months Lag

I have a question i am confused very much with i am doing on MTD Average , i really need some help with code.

the logic behind the code is



3 MTD should be the past two completed months as well as the dates in the current month as well. So taking the example above – if the reporting period for Feb was 01/30/2007 to 02/27/2007, and March was 02/28/2007 to 03/27/2007, the 3 MTD should then be the average from 01/30/2007 to 04/10/2007.



This code does not actually gives me the exact value, please advise on any correction to the code , or a new way to write the code.



Avg
(
[Time].[Month].CurrentMember.Lag(3) :
[Time].[Month].CurrentMember,
[Measures].[A1_Avg]
)