Clean and re-use. If you go buy new sand, your going to have to rinse it also. So either way you are rinsing sand.
If the sand had just been pulled from the tank, then I would say to leave it and go get new because of the threat of toxins. But the sand has been sitting for 3 months in open buckets (I'm assuming being that you said it's been drying).
Take the sand and fill a bucket a 1/4 of the way, and rinse it with a garden hose until the water runs clear. It's a tedious process, so plan on having some spare time.
If you decide against using the old sand, do not go and buy so called "live sand". Get some dry aragonite sand of your choice and then buy a few cups of live sand from the lfs from their system or from a local reefer, and dump it on top of your new sand in a pile. The bacteria from the lfs sand will migrate/spread to your new sand and then start to populate it.