Fasting means nothing by mouth from (usually) midnight on. NOTHING by mouth means not even chewing gum or sucking on a mint. You can only brush your teeth if you DON'T swallow anything.

But, with you being pregnant and with a glucose test, I'm not sure on the water. Usually, I'd say NO WATER, but...pregnancy tends to change the rules. If they were doing a 3HGTT on a non-pregnant person, I'd say no water at all. It can dilute anything you're adding to the body. They want a reading of how your body handles glucose, and I'd imagine that if you drank water, it'd effect that.

You can call a hospital lab and ask. The labs are open 24/7. Or you can talk to an ER nurse - they SHOULD know the answer...but I'd bet you'd get a better answer from the lab.