One is not supposed to take 3 steps back at the end of Tefillah if there is someone davening behind them within 4 amos. (See here for more.)
What if the Shaliach Tzibbur is finished davening, and the congregation is ready to begin the repetition of the Amidah, but there is someone immediately behind the Shaliach Tzibbur who is still davening? Does he take three steps back anyways? Does the whole tzibbur have to wait? Something else?