If you continue to run 3 x 30mins a week, you'll reach a plateau, and to maintain the fitness you have got by getting to that plateau, you have to continue with the 3 x 30mins, if you stop, you'll lose that fitness and need to do the same again to get to it.
Obviously, you'll be fitter it the 3 x 30mins plateau than you would be by doing nothing, and if you are doing 3 x 30mins and eating carefully, you'll loose weight, however, it is worth remembering that to loose 1lb a week, you need to be in calorie deficit by 3500 cals, and to do this JUST by running, you'd need to run 35 miles a week.
Bottomline here, some running is better than no running, any cardio excersice is a good thing, what YOU want from it, either just maintaining fitness and getting some excerisce, or getting faster, going for longer, or loosing weight, is up to you, but you will have to alter your training program to get your personal desired results.