A three hour trip isn’t too far. A pit stop would be recommended if your cat doesn’t travel well in the car and is becoming stressed. Always make sure they are secure and unable to escape from the car if stopping.
During the drive, ensure that your cat has sufficient water and is kept calm and safe in a secured cat carrier. The temperature of the car should be kept comfortable for your cat (and yourself if you were wearing a fur coat!). Speak to your vet about spraying the carrier and your cats blankets with a synthetic analogue of feline ‘happy’ pheromone. This can make the world of difference in stressful situations.
Diabetic animals require a strict routine with their meals and injections. This routine shouldn’t be compromised on the trip.