My 3 kids are younger (3.5, 2, 2) and our Diono Radians fit perfectly across with ample room to load/unload (better than SUV / Mini Vans we've had)
S with Pano roof can add cross bars and roof box. I have Whispbars and Yakima Skybox for trips needing more storage.
With 3 across there is a Middle Seat to fight for - this can be a pro or a con depending on how well they act like siblings.
X is incredible but if you end up with the popular 6 seat config for a less cramped feel then your cargo space is less than the S.
Even Elon has said he prefers driving his S over X but that comes to a personal preference. In my house my wife would rather be high up and in an X while I like the low to the ground sedan