how to calculate moving sum in the table

[ShaoChi]

If I have data like this:

ShoppingDate	consumer	item
1/1	A	1
1/3	A	2
1/5	A	1
1/15	A	2
1/20	A	5
1/2	B	4
1/3	B	1
1/10	B	2
1/11	B	1

I want to calculate the sum of purchases made by each consumer in 3 days, like this

ShoppingDate	consumer	moving sum
1/1	A	1
1/2	B	4
1/3	A	3
1/3	B	5
1/5	A	3
1/10	B	2
1/11	B	3
1/15	A	2
1/20	A	5

How to use "Chart General Numeric Functions" to get the field "moving sum" value ??