If there are 3 siblings, and 3 vehicles are left to divide amongst them, is there a waiting period for them to take possession in NC?

Not specifically related to the vehicles. There is a waiting period after publishing the death for creditors to file claims against the estate before anyone can legally inherit anything.

That actually doesn't prevent taking physical possession. It prevents taking legal ownership.

The value of the individual vehicles and agreement among the heirs about who gets what could also create delays.

Yes. It's called probate and the actions of the executor of the estate. The siblings can't just take things.

Yes.  The waiting period is between the death and when the estate (cars) is distributed.

The estate has to be probated first.

When a person dies, his estate gets wound up.  Presume that is what you are referring to?  His executor pays off his debts, selling his house and cars to do so if necessary.  If there is anything left, it is distributed in accordance with the will.  Typically the process takes between one and two years depending on the complexity of the estate.