Try this:
Private Sub ChangeBackcolorRed(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.MouseEnter, TextBox2.MouseEnter, TextBox3.MouseEnter
sender.backcolor = Color.Blue
End Sub
Private Sub ChangeBackcolorWhite(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.MouseLeave, TextBox2.MouseLeave, TextBox3.MouseLeave
sender.backcolor = Color.White
End Sub