we usually top it off pretty high with wood chips and then not much sprouts in it. once in a while a quick skim gets most weeds taken care of. three inches i would consider too thin. anything that lets light get through won't work well for long as most things degrade with exposure to the sun. i also don't mind stacking them deep if i have them as then we can get some mushrooms (we don't eat them, but we like them to look at).I agree with Thistle on the weed barrior. I hate that stuff! It does exactly like she says. Weeds go through it and it lifts when you weed. I will never use it again.