3 dependent textboxes in a userform

EnableEvents doesn't effect userform events, but you can get the same effect with a module wide boolean variable

' in userforms code module

Dim ufEventsDisabled As Boolean

Private Sub TextBox1_AfterUpdate()
    If ufEventsDisabled Then Exit Sub
    
    ufEventsDisabled = True
    TextBox3.Text = CStr(Val(TextBox1.Text) + Val(TextBox2.Text))
    ufEventsDisabled = False
End Sub

Private Sub TextBox2_AfterUpdate()
    If ufEventsDisabled Then Exit Sub
    
    ufEventsDisabled = True
    TextBox3.Text = CStr(Val(TextBox1.Text) + Val(TextBox2.Text))
    ufEventsDisabled = False
End Sub

Private Sub TextBox3_AfterUpdate()
    If ufEventsDisabled Then Exit Sub
    
    ufEventsDisabled = True
    TextBox2.Text = CStr(Val(TextBox3.Text) - Val(TextBox1.Text))
    ufEventsDisabled = False
End Sub