If you have an enclosed space under the floor, you need to provide an access opening.

Its just a wooden foor with 3” cavities built on top of an old garage slab. I dont need to access the space. its only 3” high. my question is, do I need to vent or can I just block off.

thank

Your own experience would indicate that you don't.

If they were just raising the level of the floor and there is another slab underneath, maybe it doesn't have to be vented.

I'm no expert so what are the reasons for venting a 'crawlspace,' and would they apply in this situation? It's 3" high so no one can actually go in. It's sealed off from the ground and the atmosphere on all 6 sides. I don't know the answer, I'm just asking.