Q: Do you need to use all 3 PCIe connectors?
A: I would stick with 3 per hashing board.  They make sure in email, and in a paper in box to tell you to use 3 PCIe per hashing board. You might get buy with it but it would void warranty.  I would say depends on gauge wire and quality of psu.  But to make life easy and in warranty stick with 3.

ALL the 3 PCIE connectors are needed to be connected to power supply on each hash board since it is based on serial power solution and there is no DC/DC inside the miner. Including the one PCIE connector on control board, there are 28 connectors in total.
Several PSU will be needed to support the miner, anyone spare PCIE connector from PSU will be connected to control board.
But be sure to power all the other PSU first and power on the last PSU which control board is connected in the end. this is very important!!