Because usually after the disconnect you need fusing for the distributed power, What I generally do is daisy chain the three phases using appropriate sized cable across DIN rail mount fuse blocks, the fuse blocks I use have two conductor entry to facilitate this, if you lay it out so that the fusing size gets progressively smaller, the corresponding cable size can be reduced as you progress across the panel.
Contactors that have O/L fitted still require fuses before them, but just the one O/L.
Do not use a ground as a feed conductor, if only 3ph is entering the panel, then wire a transformer across 2 ph with appropriate primary, with 120v secondary.
It is customary to ground one of the 120v secondary leads to set up a local grounded neutral, I have previous posts here on how to accomplish this.
If getting into machine control wiring, it is worth springing for a copy of ANSI/NFPA78, it runs around $40.00.
This also explains the local neutral I mention, as well as Electrical Code.