I'm not sure what you mean by an 'aspire 3' coil, but the best wattage setting will depend on the coil. Coils usually have recommended power settings written on them, or at least on the box the coils come in. If you can find the recommendations, start at about 1/2 the lowest setting. Then increase the setting gradually. The vape should improve. Keep increasing until the vape starts getting worse, not better, then turn it down to where you liked it best. That's the right setting for you.