What size is 00 crochet hook in US?

00 is a steel hook size. Manufacturers vary in their mm sizing.

I found 00 hooks listed as 2.75 to 3.5mm

If you have a particular hook and you need to know the mm sizing of it, I suggest you purchase a Crochet Hook Size Gauge Tool. The knitting needle ones do not give accurate sizing for a crochet hook. Annie's Attic sells these and I'm sure other stores do as well.