J.R.
3 hours is not bad at all, although you might want ot make it a 2 leg trip with a stop at a park or rest area in the middle, especially if it is their very first longer car trip. We have been taking our kids to West Virginia and Tennessee a couple times a year for each place, from Central IL, since they were each about 5-6 months old. West Virginia is about a 12 hour drive for us and we have done it successfully with a 1 and 3 year old in one day - just have to make sure to stop every 2-3 hours for a short play/potty/eating break.
Make sure to pack snacks, and drinks in a cooler. Bring some toys they either haven't seen in a while, or buy some new ones for the occassion ( I save up happy meal toys ina secret bag in the car for this). Bring the 3 year old a lap desk and crayons and a coloring book or paper. Constantly talk with them from the front seat, asking them to look outside for certain things, barns, animals, cars of particular colors, motorcycles. Bring books to read to them. Bring some fun CDS of kiddy songs to sing along to.
Like others said, start driving right at the beginning of naptime, or right before bedtime, and they could sleep for most of the trip - make sure you wear them out playing outside or something before you leave. Make sure to bring any security items, for the car and for sleeping away from home - blankie, nightlight, any sounds they are used to - fans or music.
We do have a portable DVD player form little tykes for our 3 year old, but we only use it incase of emergency, like in the 12 hour of the trip to WV when she is just to tired for anything else to be entertaining - we wil be giving one to our little guy when he turns two, as well, but same thing - only when we are to our wits end, we don't want them expecting to veg to a movie the whole time ( that is why we didn't get the integral DVD system when we got our new van - didn't want it to be a fight/expectation every time we were in the car).
I heard of a family who put a clothesline across the backs of the front seat and marked with different colors where each 100 miles or 1 hour or whatever milestone passed, and would move a clothes pin with a little car glued to it, along the line to show the kids how far they had gone, and at each milestone, the kids got to unwrap a little dollar store prize.