Level three keyspheres arent' what you should worry about. I'm sure you can get several in the Omega Ruins.

Be saving your gil for teh level four ones. Bribing chimera brains is just about the only way to get enough.

For some reason, getting a level 4 keysphere before a level 3 seems about right. Just don't go too far away from where you need to go. You'll get some eventually. They like to give them out via bosses.