How to I talk to my 3 yr old adopted son about the death of my paternal grandfather? My son only met him a few times FWIW.

If he’s already aware of the death, I’d say, rather than telling him something, start by asking him something. Let him guide the conversation. You may find he’ll be curious or concerned about things you aren’t thinking of.

If you’re breaking the news to him, though, do that gently (and keep it simple), then see above.