How much should be the trace width on PCB for max. 15A from 3 Phase AC Source?
You have two crucial issues here:

Current handling (track ampacity) and
Insulation (track spacing)

The first one is easy, there are graphs/table to design that. You didn't say how much copper are you using if 35µm or 70µm, this is very important. The relevant standard is IPC-2152 and you can find online calculators using that (a first approximation would be 5mm of width)
The second (arguably more important) is insulation, i.e. spacing (clearance) between traces. You didn't see which overvoltage CAT you circuit is subject to, so it is difficult to say. Usually you want functional insulation between your 400V lines and reinforced insulation between 400V and the low voltage section. There is also another parameter (called creepage) that's an adjustment for the distance. All of this depends also on the environment the board is used in (usually called the 'pollution level'), there are many EN standard covering the requirements. I gave you many keyword to search for.
These are safety features, do not attempt a design if you haven't completely clear the requirements. You do not want your 3-phase supply leaking on your low voltage section. Grounding is critical, too.