Consolidate Multiple Columns In Multiple Worksheets

I am trying to consolidate information from multiple (atleast two or more) worksheets (in the same workbook) into a single sheet.

The worksheets have identical column headings. There are 4 columns in each worksheet. The first three columns are text strings (and some cell in these columns may be blank). The fourth column is an integer. The number of rows in each worksheet is most likely to vary. Within a worksheet the rows are unique. However, some rows may be duplicated across worksheets.

To reiterate, here's what I am trying to do.

Step 1. Create a summary sheet.

Step 2. When there is a unique row (identified by the first three cells in that row) across worksheets then I have that row as is in my summary sheet.

Step 3. When there are duplicate rows (identified by the first three cells in that row) across worksheets, I want to add up the cell in the fourth column associated with those row in my summary sheet

I have tried union (works only within a sheet), consolidate (works only with a single column). I don't need the sophistication of a pivot table but simply an aggregation of data

I am trying to do this using macros (within an add-in)

I have enclosed a sample workbook with the expected output. I created this workbook by hand in an attempt to clarify my problem statement.