VBasic 2008

Mediante la propiedad changue del txtbox, cada vez que hagas cambios en uno, haces que el texto de los otros sea igual al texto del que estas cambiando

VB2008
Public Class Form1

    Private Sub TextBox3_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox3.TextChanged
        TextBox2.Text = TextBox3.Text
    End Sub

    Private Sub TextBox2_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles TextBox2.TextChanged
        TextBox1.Text = TextBox2.Text
    End Sub

    Private Sub TextBox1_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
        TextBox3.Text = TextBox1.Text
    End Sub
End Class