Each gummi route's Mission 3 can be rated up to S+10. Mission 3 is unlocked based on the route's Mission 1 and Mission 2 ranks.
The old S+10 sticky topic on this board was unsticked long ago, if that's what you're remembering, before topics archived. Gone with the Schwinn.
See the KH2-FM guides for info on the differences in gummi routes, rankings, and treasures. I'd start with starrk's big Translation Guide m'self. --- Guides and other contributions: http://www.esque.com/slr/gamefaqs/ FFX, FFXII, KH, KH Re:CoM, KH2, Okage: Shadow King, Secret Agent Clank