The "fancy" seat option (standard on Founders and sig) for the X offers 4 LATCH points for car seats -- the outboard 2nd row seats and both 3rd row seats. If you know anything about LATCH, you can spot the tell-tale plastic circles on the seats shown on the main Model X page.
However, just because there aren't LATCH points on the 2nd row center seat doesn't mean you can't put a car seat there -- every car seat that I know of can also be secured "the old fashioned way" via the seat belt. Just make sure you figure out how to use the metal seat-belt-locker thing that comes with the car seat.
Further, if I remember correctly, there was a message from Tesla at some point saying that the "base" seats for the X would offer 5 total LATCH points...3 in the second row and 2 in the 3rd row. I'm guessing we'll get some hint of those when Production models get to configure.
Finally, if you are determined, and creative, you can fit 3 car seats in many cars. While we do have a Honda Odyssey for our 3 kids, I have also managed on a few occasions to get 3 car seats in the back seat of my commuter: a 2008 Honda Fit. All 3 seats were well secured and all 3 kids were comfortable (5 year old in full booster including the backrest part, 3 year old front facing in a huge carseat, and <1 year old rear facing). Based on that, I can pretty well guarantee that 3 car seats will fit in either an X or an S just fine.