There's no reliable way of getting player count. But yes, you'd need target_count and target_stgctrl.
(Actually there might be a way: use LOTS of nodes in 2 places, 1 place where ppl spawn at the first time they join team, then they get counted & teleported to 2nd area, where after death they will spawn from 1 of closest nodes, which does not recount them. But you need to know how the first node is chosen (perhaps closest to XYZ=0 0 0? post if you find out please). To use player count, set the count to for example 100 (can be done with a loop which is stopped by another count) then count down by Y from that for each player.)
And Kiwi: if someone wanted s3 in every map, it should go to mapconfigs/default.cfg. Otherwise map votes would not give s3.