Three’s company! Kourtney Kardashian invited Scott Disick’s girlfriend, Sofia Richie, on a family vacation with their three kids, Mason, 9, Penelope, 6, and Reign, 4, to Cabo San Lucas, Mexico, over the holidays. The 39-year-old reality star and her one-time nemesis, 20, “have a lot more in common than they thought. Of course, there’s some unspoken competition between the two, like, who’s got the cutest bikini or the best body,” a source tells Life & Style. “But Kourtney and Sofia are trying their best to make it work. Scott looks like the happiest guy in the world right now.”…