While "s hift happens" as the Southwest flight attendants say, I have actual experience with the fact that....PROPERLY INSTALLED there should be no seam issues.
My table is in a converted garage which is not tied into the house's HVAC system and instead, has a through-the-wall heating/AC system....like a motel room.
Therefore, the pool room is subject to significantly greater heat/cold/humidity variations than a room that is on a central system.
But even on central systems, where people use cost saving temp settings at night or when they are out of town, a poorly installed 3 pc.
On two occasions, I had meaningful buckling of the slate causing the visible seam. But I finally found an excellent mechanic who uses something other than plain beaswax. If memory serves, it was some combination of beeswax and a Bondo-type substance.
He guaranteed against seam shifting and after a year, he is absolutely right.
So, if you are considering a 3 pc. system, I would absolutely ask the installer what seam substance he uses and ask for a written guarantee against seam lines.
Regards,
Jim