It's impossible to say for sure without knowing how much you weigh and how long you rode your bike. A guess would be 100 calories.
Three miles is a very short ride on a bicycle. If you are just starting out, then that is a good start. Try to work your way up to 5 miles. It doesn't matter how fast you go, but try to go fast enough to feel that you have to work. When you finish a ride you should feel that you have really exercised your legs.
You might want to get a cheap bike computer that will keep track of your mileage and speed. That way you can keep a consistent pace each time you ride and you will know exactly how many miles you have ridden.
Good Luck