Relationships in 2019 ...Knock On Their Door: Scott Disick Takes His Ex-Kourtney Kardashian & Girlfriend For Vacation

Jack, Janet and Chrissy got nothin' on this trio -- Scott Disick, Kourtney Kardashian and Sofia Richie looked tight as can be in Mexico for the holidays.

The threesome are down in Cabo San Lucas right now enjoying themselves a beach day with Scott and Kourtney's kids, and Sofia tagging along ... which she's done from time to time. 

It's a pretty incredible scene -- Kourt and Sofia are seen lying up on the some chairs, side-by-side ... and they even go on a stroll with Scott at one point on the sand. No signs of tension, hostility ... nothin'. Just your run-of-the-mill modern day co-parenting at its finest.