New to website... How to calculate the sum of last 3 months rolling over each new month!

Hi I am new and I have been trying to work out my problem for so long I am pulling my hair out.

I am trying to calculate the sum of the last 3 months when new data is added every month without having to change the formula.
Like this;
For the last 3 months
Aug-13 5
Sep-13 4
Oct-13 4
Nov-13
Dec-13
Jan-14
Feb-14
Mar-14

Then when I add data it calculates the sum for the last 3 months - now leaving august out and moving down to november
Aug-13 5
Sep-13 4
Oct-13 4
Nov-13 4
Dec-13
Jan-14
Feb-14
Mar-14

I need it to be laid out how I have laid it out in my excel spreadsheet.

I have tried the offset function and it does not work and other formulas only calculate sum of one month.

I have also tried to manually input formula but it doesn't work either.

I have attached the data and I cant change the layout because this needs to be spread company wide. I need the figure for jobs to appear in K6 and the figure for CVsent to appear in L6 an so on.

Imagine I am adding new figures every month.

I know the formula is something like =sum(3months)-previous month but this has to be running.

I have attached the data. Please help. I hope I am making sense.