From the code it appears that you are using only the first item in the selected items.
You can loop through all the selected items as follows
Dim strHeap As String
Dim strObject As String = "C:\Temp\ASERVICE.EDM"
For Each item as ListViewItem In Radia_Listview.SelectedItems
strHeap = item.Text
SetZAVISValue(strHeap)
System.Threading.Thread.Sleep(10)
SetDATEValue(strHeap)
System.Threading.Thread.Sleep(10)
SetInstallValue(strHeap)
Next