If you're performing a vigorous-intensity activity in those 30 minutes each time, then you would be easily reaching the 75 minutes of intense exercise recommended per week. Whether it's enough to reach your goals depends on your fitness levels and what those goals are, but it's undoubtedly a good start. However, if you are doing a moderate cardio activity, like a walking workout, just three times a week then that falls short of the 150 minutes recommended per week.  Ideally, you should adopt a balanced workout routine with both cardio and strength training, as well as following a healthy and balanced diet.