Why do Catholic laity cross themselves three times before the Gospel is read?

Why does the laity (and sometimes the priest) cross themselves 3 time before the Gospel is read? Like holding hands during the Lord's Prayer, and unlike bowing at the mention of the Incarnation during the Creed, it's not something that is called out in any of the missals or Mass cards I've ever seen in the pews, but it seems to be a pretty common practice; at least in my diocese.