Cell Function to Monitor a Specific Range

You can't do what you are describing with a worksheet function, Cell or otherwise.

You would need a VBA Change event handler monitoring changes to the specific range you require. Ideally, that range should be kept to a minimum.

It would look something like this:

' Module: Sheet1 Class Module

Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)

Dim rInterest As Range, rValue As Range
Set rInterest = Me.Range("A1:C20")
Set rValue = Me.Range("A1")

If Intersect(Target, rInterest) Is Nothing Then Exit Sub

Application.EnableEvents = False
If Target.Cells.Count = 1 Then
    rValue.Value = Target.Value
Else
    rValue.Value = "Multiple changes"
End If
Application.EnableEvents = True

End Sub

Regards, TMS