Better all at once?

I have heard either option is good, and that it doesn't make a difference if you do it all at once or break it into smaller bits throughout the day.

I think it depends on how fast you run. No, seriously. You get the most benefit out of aerobic exercise if your heartbeat is elevated for at least 14-20 minutes (benefits including an elevated metabolism for an extended period of time post-exercise). Unless you're jogging at a fairly slow pace it won't take 20 minutes to go 1 1/2 miles. So while you'll certainly benefit from running 3 miles every day, it won't be the same benefit splitting it up vs. doing it all at once. But if, rather that doing a fixed distance, you ran for 20 minutes twice a day, then you'd have the best of both worlds.

I hadn't heard that, Kerri, but after reading your post I looked around. The Mayo clinic agrees with you. . .here's an excerpt from their site:

Longer, less frequent sessions of aerobic exercise have no clear advantage over shorter, more frequent sessions of activity. Any type of aerobic activity contributes to cardiovascular fitness. In fact, even divided "doses" of activity — such as three 10-minute walks spread throughout the day — offer aerobic benefits. What's most important is making regular physical activity part of your lifestyle.

For healthy adults younger than age 65, the American Heart Association and the American College of Sports Medicine recommend at least 30 minutes of moderate-intensity aerobic activity (think brisk walking or swimming) five days a week or at least 20 minutes of vigorous aerobic activity (such as running) three days a week, plus strength training exercises twice a week. For adults age 65 and older and adults who have chronic health conditions, balance exercises are recommended in addition to aerobic activity and strength training exercises. If you want to lose weight or meet specific fitness goals, you may need to increase your activity even more.

The bottom line? The more active you are in general, the greater the benefits — whether you choose longer, less frequent workouts or shorter, more frequent workouts.

In the warmer months I commute by bike, it takes about 25ish minutes each way. I do notice a difference in performance and wieght loss after a while of this, so I would say you are getting at least as much benefit from breaking up your workout as doing it all at once...