Three Phase Rectifier Question (student)
The problem is you are shorting the supply. 

What you have is the NEUTRAL of the supply (associated with the SimPowerSystems reference) and the NEGATIVE of the rectifier commomed together. Essentially all phases are shorted via one diode.

In practice a rectifier will produce a mean voltage who's differential voltage is \$\ \frac{3\cdot \sqrt{6}}{\pi}\$ that of the RMS,for 3phase systems thus a 230V phase ~ 540Vdc, DIFFERENTIAL. The common-mode voltage with regards to the NEUTRAL of the supply is +-270Vdc (for a 230V phase).

How to fix?  leave the rectifier negative floating.