What is capacitance required for this 3 phase rectifier
One of the big advantages with 3-phase supply in this situation is that even with zero capacitance you get a good DC supply with no drops to zero volts.



Figure 1. Three-phases, half-wave rectification and full-wave rectification with DC value superimposed. Source: Wikipedia. Click image for high-res view.

For a three-phase full-wave diode rectifier, the ideal, no-load average output voltage is

$$ V_\mathrm {dc}=V_\mathrm {av}=\frac{3{\sqrt 3}V_\mathrm {peak}}{\pi} $$

Adding capacitance may help with power glitches and transients. You probably don't need any.

You can't connect neutral in this situation. Neither the DC+ or DC- are at neutral potential (voltage).