Babies in the 3 month old range sleep most of the time. If it is just the two of you, then make sure you position the car seat so you can reach back and comfort the baby if he becomes fussy. You can attach musical mobiles to the car seat to keep your baby entertained. I would pull over (in a safe area of course) about 15-30 minutes before feeding time, so you can stretch a bit and prepare the bottles if you are need feeding naturally. If your baby becomes really fussy, then you want to pull over and take him or her out to get a bit of fresh air and to feel the comforts of your touch. Most 3 month old as long as they are fed and dry they are usually content.
Long drives with an infant can be pleasant if you prepare yourself to take frequent breaks during your journey. Plan to take break after every 2.5hrs-3 hrs to change and feed the baby. Also a little fresh air and different position (personal touch) during the break will cheer up the baby. Also while driving, stock yourself with some musical toys, clothes, diapers, formula and extra bottles.
Taking an 8 hour car trip with a baby can be pleasant if you prepare. Make sure you take a break every couple of hours for changing and feeding. Shade screens installed on the windows will also help to protect the baby from the sun. To be on the safe side, keep your child in their car seat at all times except when you are stopping to take a break. A first-aid kit may come in handy also for those little minor mishaps that may occur. Also, bring plenty of diapers, baby wipes, and formula. A small blanket can come in handy for changing on the backseat.