Retrieve values from multiple sheets and use VBA Sum to place total on Main Sheet

Hello,

I have a workbook with 7 sheets. 1 of the sheets is titled 'Metrics'. 4 of the sheets have a value in D3 that I would like to have added together and placed in C3 of the Metrics sheet. I'm open to placing a formula in the sheet and calling a function too but that didn't work so I moved to this method. I have tried many variations to sum. Here is my latest catastrophe. Any idea how to resolve it?

This VBA is on the metrics sheet.
Sub TotalTestCases(Range)
Dim ws As Worksheet
Dim SumTotal As Long

For Each ws In ThisWorkbook.Worksheets
If ws.Name <> "Metrics" And ws.Name <> "TFSData" And ws.Name <> "TFSBugs" Then
SumTotal = WorksheetFunction.Sum(ws.Range("D3"))
Metrics!C3 = SumTotal

End If
Next ws

End Sub

oh, and I would like them to automatically update if the value on one of the sheets change