As before, the Runner is only available on first playthrough unless another character is unlocked by other means.
Level 3, like the previous two levels, is still very easy. It introduces crumbling tiles, which break and fall when stepped on. There are still plenty of normal tiles to run on, however, so this shouldn't be too much of an issue. If you dislodge a crumbling tile and start to descend be sure to jump or maneuver away from it. If there are any connected crumbling tiles they'll all fall once stepped on, so watch out.