I'm quite uncertain I understand your request, but anyway here should be some hint.
If possible, please attach your packaged workbook (***.twbx) for us to study further more.
I created sample data, in this case, we want to sum up 2016/1/1~2016/3/1.
The answer should be 2229 which is show as Running_sum on 2016/3/1.
At the same time I added [First] calculated field as index.
Now when you try to get the number of 2229 without any dates on column, you can put the formula like this.
[First Three Values]
Window_sum(if [First]>=-2 then sum([Value]) END)
Then the number is shown in worksheet 3.
with unchecking Show header of Start Date and filtering only 0 for "Firs" as filter.
First three Value_SM_9.1.twbx 20.9 KB