Yep...supposed to be 3. Two big o-rings...one of which is real thin, is placed at the upper end/top of the cap and isn't strictly needed. The other a lil thicker and fits in the groove and is needed....and of course the lil one on the bolt.
Sometimes the thin one will break and if you aren't paying strict attention when the cap finally pops off, may land in the hull. Or...and a real possibility...was never there to begin with.