Three pallets of gray slate floor tiles. What to do with them?

Donate them to Habitat for Humanity.

https://www.habitat.org/restores

make cheese platters and sell them..I love slate for cheese serving

How about donating them to Habitat for Humanity?

Use them as pavers in your garden, make craft projects out of them...trays, cheeseboards, etc. Cover an old concrete patio. Lots of great things you can do since you have a lot of them.

If you have more than you can use, you might want to consider putting them on Craigslist. If you just want to get rid of them as soon as possible, find the closest Habitat ReStore. They accept donations like this. This store sells home furnishings, hardware, and DIY types of materials. And while you are there, you might find something you want in your own decor!

Habitat ReStore proceeds go to Habitat for Humanity.

Hi Matthew, we made a whole bunch of heart slate boards and sold them at a craft fair. They were very popular. Here's the tutorial on how we did it - https://acraftymix.com/blog/slate-serving-board/