Record Calculated Value Before Changing Inputs?

A1 = 5
A2 = 7
A3 = A1+A2 = 12

Now I would like to keep value of A3 in cell A7 every time I change input values in A1 and A2 and so on.

A1 = 15
A2 = 8
A3 = 15+8 = 23
A7 = 12

And when an input in A3 > 100, it will popup a message, that a total cannot over 100. My code:

Private Sub Worksheet_Change(ByVal Target As Range)

If Target.Row = 1 And Target.Column = 3 Then

Range("A7").Insert , CopyOrigin:=xlFormatFromLeftOrAbove
Range("A3").Value = Range("A7").Value
Exit Sub
Else
If Range("A3").Value > 100 Then MsgBox "Total exceed 100. Check you inputs!"
End If
End Sub