So this is going to contain spoilers. To make sure it doesn't appear on the front page, let me say I loved the movie and thought it was a blast. Superior to the second one, as good as the first one. And stay for the after credit sequences.

Good. Now my question is: When Killian says he was the Mandarin from the beginning, what did he meant? That he was behind the Ten Rings group in 1 and 2? Or was he just revealing what we already knew, thinking Tony didn't know by now?

The presence of the Ten rings in this movie without any link to the first one (and it's brief appearances in the second one) was problematic, but not too much. The easiest way to explain it would be that the group died after all its leadership got killed in the first movie, and Killian stole its iconography.