I'm pretty sure they don't. They eat detritus, left over food and stuff like that. Mine bury themselves in the sand and only come out when I feed the fish or at night. There are no real rules with livestock, just buy a few and if after a little while you think the tank could do with a few more then just get some more.
Depending on the tank I don't think one per 3g would be overdoing it. But like I said there is no real rule to it. I think my nassarius do a pretty good job at keeping the sand clean, I have 3 in a 20g.