Hi TLNTX,
Their standard king size is 75" x 80" and a “normal” dimensional tolerance is about 1" so 3" would be a little more than a “normal” variance (see post #16 here).
I’m not sure how long the mattress has been decompressed but it’s possible that it will regain a little more width over the course of the first few days … especially if it was compressed for a few weeks.
I would also make sure that you are measuring from the widest part of the side panel on one side to the widest part of the side panel on the other rather than from the top corner to the top corner because with a new mattress the tightness of the cover can pull the corners in to some degree.
Other than that I would talk with Dreamfoam to see what they suggest as well (and hopefully let us know what they say).
Phoenix