Absolutely not! If you have any camping experience, and plan accordingly, it's not at all unrealistic for you to do a 3 day for your first time out. As long as everyone has the correct mindset (it only takes one person to infect a group to want to paddle through and finish early), 3 days is a magical experience!
TIP: Cooler management! If you're going to be on the river for three days you really need to think about cooler management. We HIGHLY recommend marking your coolers. Cooler #1 will be used from the start of your trip to roughly the halfway point. Cooler #2 is lashed closed and not opened for any reason until you get past the halfway point of the trip. Coolers that get opened don't remain "coolers" for long.
TIP #2: Freeze as much of the contents of your cooler as you can - it will keep the entire cooler colder longer.