The current National Activity Guidelines for Australians recommend that you should perform at least 30-mintues of moderate exercise every day. The exercise should be intense enough to increase both your heart and breathing rates. For greater health and fitness benefits, the guidelines also recommend 3-4 sessions of vigorous exercise per week for at least 30 minutes each time, in addition to the moderate exercise listed above.
If you are only exercising three times a week, then this is better than nothing but for optimal health and well-being you should aim for at least 30 minutes a day, every day.
If you would like to view the National Activity Guidelines they can be found at – http://www.health.gov.au/