We only had a 45 minute drive with Denna. For 3 hours, I'd plan on at least one break in the middle for a stretch and potty. As mentioned on other threads - you might want to avoid other places where strange dogs have been (i.e. the dog-walking area at rest-stops) - just to avoid any unwanted germs before you get all your vaccinations in place.
We put Denna in a crate with a blanket from our house, and the blanket that had her mom and littermate's smells on it from the breeder. She whined a bit, but we did our best to ignore it or say "shhhhh" and "quiet" very, very calmly. After about 7 minutes she went to sleep and napped the whole way home. She was really good in her crate at night as long as she had her woobie (the breeder's blanket). We took her out pretty much every 2 hours for the first 5 or 6 nights home, then it was every 3 hours, then 4, then... 8!!! Only about 2 weeks or less of the mid-night potty breaks (unless she had an upset tummy from too many treats or a change in food...). We always took her outside if she whined in the crate at home - out for potty, then back in. Period. She learned really quick that the crate was for sleeping, and never really complained about it.
Hopefully you little one will be as good as ours! I'm still in amazement at how good and calm of a puppy she's been.