I've never heard of the three sips rule. (Source: Huffington Post today).
Edit: I suspected this was bogus (that's why I posted it), but I've been leery of Texas alcohol laws ever since a Wise County deputy called my attention to some crazy bootlegging prohibitions that we have. By the way, some folks say the "three sips" law is actually an ordinance in Lefors, Texas (but I wouldn't bet on that either.)