Is it better (more effective) to exercise for several short periods a day or for one or two long periods, if it all totals up to the same amount time?

It doesn't matter whether you do a one 30-min workout or divide it into three 10-min workouts. As long as you get at least 20-30 minutes total each or every other day, you'll be fine.

The following is IMHO:

I assume that by exercise you mean cardio.

If that is the case, I am going to have to disagree with the idea that 10 + 10 + 10 will give you the same result as 30 minutes of continuos exercise. Read on...

A calorie is not a calorie. Experts say that after 15-20 minutes of continuous activity the body will metabolize more fat for fuel. Anything under 15 minutes is usualy not from the fat stores.

However 20 in the morning and 20 in the evening is okay cause both sessions exceed the recommened time for fat loss.

I do recommend doing High Intensity Interval Training instead. In about 15-20 minutes you use up more energy than steady training and you have more fun.

Check this link out for more info:
http://www.musclemedia.com/training/hiit.asp

Loon