Yeah, do you have any currently running SW tanks, or will this be your first one?? A 3G pico reef tank will be exceptionally hard to keep stable, and corals are not like FW fish, any rise in nitrates or a slight change in specific gravity or chemicals can spell death. I would go for at least a 10G if this would be your first SW tank. And, no fish in a 3G - perhaps a shrimp and some snails, but that's it.