Public Class Form1

    Private str As String

    Private Sub btnA_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnA.Click
        str = TextBox1.Text & "a"
        TextBox1.Text = str
    End Sub

    Private Sub btnB_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnB.Click
        str = TextBox1.Text & "b"
        TextBox1.Text = str
    End Sub

    Private Sub btnC_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnC.Click
        str = TextBox1.Text & "c"
        TextBox1.Text = str
    End Sub

End Class