Tell your vet you need sedation for the cats. The sedation should be mild, of course, but it will help them acclimate to the new situation quicker…and they will stop howling after an hour or so.
Put them in their designated carriers, with good airflow. There are small ‘clip-on’ fans you can buy that jack into the 12-volt recepticles in your vehicle. Use one or two to keep the air moving around your kitties’ carriers. I wouldn’t put them in one carrier, as it means three agitated bodies in one space (heat). Put something familiar…a toy, your worn T-shirt…in each carrier.
Having the cats loose is just inviting serious trouble. A cat under the brake pedal is inevitable. One crawled inside the dash can be expensive and hard on the cat. Let us not even discuss the possibility of a kitty escaping the car while stopped many miles from home. It is safer for everyone if the cats are contained.
If possible, put catnip in the carriers, even feed them there for several days prior, to give kitties a chance to get acquainted and comfortable with them before the long drive.
Give them potty breaks, while in suitable harness at quiet areas on the road. A litterbox kept in a big trashbag is best. Kitty will recognize the box for his potty, and use it if necessary. Asking them to use it in the vehicle is a big mistake; the smell LINGERS for hours.
Give them access to water at least every couple of hours. A very small allowance of food once the worst of the sedative wears off might be okay.
Chances are they will sleep the entire trip…its only 12-14 hours, right?
I speak from experience, having traveled extensively with feline company. From Kansas to New Hampshire once…the cat was great. It was the TEENAGER I wanted to club!