You can use a module in the Worksheet "SelectionChange". Right click on the worksheet and choose "View Code". Paste this code into the VBE. Do this for each worksheet where you need it done.
Code:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rng As Range
Set rng = Intersect(Target, Range("B3"))
If rng is Nothing Then
Exit Sub
Else
Dim cl as Range
For Each cl In rng
.....
Next cl
End If
End Sub
Instead of having just Range("B3"), you could use Union of several cells, and then it would be combined into one.