3.5 miles can be done on just about anything. You may want to look at bikes classed as "city" or commuter bikes. An internal gear hub with 3 or 8 speeds will probably be good for you. Look up the Schwinn Coffee and Cream to see what I am refering to. The internal hub is cleaner and requires less maintenance than derailers, and can be shifted while you are at a stop. An electra Townie may work for you too, there are a lot of options on that one.

If you are on a bike with front and rear gears, just leave the front on the middle ring until you are comfortable and shift in the back. Once you get used to riding, "1" on the front is for going uphil, "2" is for the flats, and "3" is for downhill.