They're thrown weapons and there's no special rule for it, so, by default, you can throw one per attack you can make in the round.
Since they're so tiny, I'd see nothing horribly wrong, conceptually, with throwing all three at once like a set of shurikens. Of course, you'd only get any damage bonuses due to STR, point blank shot, etc, /once/, as with shuriken. I'd seriously consider changing the durration to 1 round, though, if I allowed that, so you couldn't go pre-creating dozens of the things and tossing hadfuls of them. If you wanted to create them as sling-bullets, the regular durration'd be fine, though...