1. Pack them so they don't get jostled around, shaken, or broken. I've used a regular egg carton. A bucket or other container would also work. You might put a small towel or something in a container to help cushion them, but as long as they cannot roll around they should be fine.
2. Keep them out of direct sunlight. The worry is they might get too hot on a 3 hour trip.
3. Don't let them get too hot or too cold. I'd aim for somewhere between 45* F and 75* F. Remember how hot a car can get if it is parked in sunlight for a while.
4. Don't let the heating or cooling system blow directly on them.
5. For that length of time, I would not worry about packing them small side down. I don't think it really matters for such a short time. However, if the way you are carrying them allows it, I'd put small side down just as a matter of good practice.
6. I would not carry them outside in the elements, like the bed of a pick-up, unless they were packed in a way to keep them out of direct sun, wind, and possibly rain and the ambient temperatures were OK.
7. Don't try to overthink it or overdo it. I'm probably making it sound harder than it is. Putting them in an egg carton on the floor of the back seat should take care of most of these worries if you have a car. Putting them there in a small bucket or plastic container without worrying about putting the small side down would also work. If you have a vehicle without a back seat, try to find a place not in direct sun where the heating/cooling system is not hitting them, and they won't get broken if you hit a pothole or have to stomp on your brakes.