Sorumun Cevabını buldum. (alıntıdır)

Public Function Toplam(ByVal txt() As TextBox) As String
Dim o = txt.Where(Function(f As TextBox, d As Double) Double.TryParse(f.Text, d) = True).Select(Function(f) Double.Parse(f.Text)).Sum()
Return o.ToString("N2")
End Function


Private Sub TextBox3_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox3.TextChanged, TextBox2.TextChanged, TextBox1.TextChanged
TextBox4.Text = Toplam(New TextBox() {TextBox1, TextBox2, TextBox3})
End Sub