How do you relax a cat?

You can't - cats don't take well to long-term sedation like what you would need to cover a 3-day trip. The best option is to take it easy while driving - no sudden acceleration or slamming on the brakes, no sudden turns, etc. If your cat is huddling in its carrier, try draping a sheet or blanket over it to shut out the visual stimulation of the scenery flashing by. If your cat is bouncing around, try leaving the blanket off and let your cat watch as the world goes by. Take frequent rest stops (every 2-3 hours) and let you cat have access to the litter box during these stops. Keep a small amount of fresh water available at all times and offer small amounts (like a toddler's handful) of kibble at rest stops.