So you don't get scared or confused by those choices, the best event is probably the TextChanged event, to which you get by double-clicking the textbox:
Private Sub TextBox3_TextChanged(sender As Object, e As EventArgs) Handles TextBox3.TextChanged
Me.TextBox4.Text = Me.TextBox3.Text 'Have the same text
Me.TextBox5.Text = Me.TextBox3.Text 'Have the same text
End Sub
Or you can have it reversed for fun:
Private Sub TextBox3_TextChanged(sender As Object, e As EventArgs) Handles TextBox3.TextChanged
Me.TextBox4.Text = StrReverse(Me.TextBox3.Text) 'Have the reversed text
Me.TextBox5.Text = StrReverse(Me.TextBox3.Text) 'Have the reversed text
End Sub
Only in case of a palindrome the output is the same for either code (as the attached picture shows).