16 AWG wire is rated for a maximum of 18 amps (or 216 watts). An S7 will have a total consumption of about 1,350 watts.

What I would recommend is using three 600 watt power supplies, with at least two sets of 12 AWG wire going from the power supply to the hashing board.

With two 1200 watt power supplies (I believe that is the rating running only on 240 volts AC), you could use one for 1 hashing board + control board, and the other for the 2 other hashing boards, but you'll want to have four sets of 12 AWG cables on that 2nd power supply to the two hashing boards.

16 AWG wire is way too small and you should not use any Y-splitters.