Just put three halves on each rack. There I also the coil,roll the rack stick a skewer or two if needed to keep the meat apart if you want them whole. I normally cut a few bones off and wrap them in bacon.
I had the same experience with a package from Costco this summer. Thought it was 2 but it was 3. No problem fitting them into the offset, but we had serious leftovers. No problem, though. Ribs reheat just fine [emoji]128512[/emoji]
I'm sure you'll make it work..