We have three remaining 6' by 6" fence pickets that have been treated for termites. My question is how best to store these?
Put them in a shaded, dry place, with an inch of airspace between them, and between them and other things.
For example, I have some fence slats under my porch. They are supported by short 2x2 pieces of wood (so that they are 1½" above what is beneath them), and have a gap separating the slats in any given layer of the stack.
You can similarly stack lumber you plan to use, or firewood that you need to dry out. But make sure that you do not store treated wood with firewood! You do not want to risk accidentally burning treated wood.
I am guessing you are keeping them as spares and do not want them to be rehabilitated by termites ?
Keep them away form high humidity areas, off the ground and away from moisture.
You can wrap them in a clear bag (termites like the dark) so make it clear perhaps that painters floor spill protection plastic. Seal it up completely with tape. Termites have no trouble with 1/16 inch opening - so make sure you seal it. You can then store it as per my previous information.