limey

If you only want to work out 3 days a week I would rest after each day you workout. If you put everything you have into each workout then you will need to rest the day after regardless. You need to remember that you will put a strain on your nervous system as well as the muscle group you are working on so resting that is just as important. Also try and incorporate exercises that work more than one muscle group at a time, this way you can hit more muscles with less exercises. Here's what I would do, but you should do whatever works and is comfortable for yourself;
Day 1: back/biceps/abs
Day 2: rest
Day 3: chest/triceps/shoulders
Day 4: rest
Day 5: quads/hams/calves/abs
Day 6: rest
Day 7: rest

You can pretty much move your abs around but, like Rdr4evr said, you should limit them to 2 or 3 times a week and work them slow and hard like any other muscle group.