If myListView.Items(17).SubItems(3) Is Nothing Or myListView.Items(17).SubItems(3).Text <> "" Then
Or if you need to check all:
Dim emptyFieldExists As Boolean = False
For Each item As ListViewItem In myListView.Items
If item.SubItems(3) Is Nothing Or item.SubItems(3).Text <> "" Then
emptyFieldExists = True
End If
Next
If emptyFieldExists Then
End If