3 Phase calculations
Formula for the line voltage.
  


$$ V_{ph-ph} = \sqrt 3 V_{ph-n} $$


  
  Formula for the Average and RMS outputs of 3-phase full bridge rectifier.
  


$$ V_a = \frac {3 \sqrt 3}{\pi}V_m = 1.654 V_m $$

where \$ V_m \$ is the peak phase to neutral voltage.

$$ V_{rms} = 1.655 V_m $$

which is very close to the average value.

The tutorial paper by Dublin Institute of Technology - Three-phase rectifier looks like it should be of considerable help to you.


  
  If I use this output of the bridge rectifier as my DC Link voltage to feed a 3-phase full H-bridge and drive it with 3-phase sinusoidal PWM then what will be Average and RMS voltages of the resulting 3-phase sinusoids?
  


Average of a sine is zero.

Ignoring losses the maximum phase to phase output voltage is the DC-link voltage. That will be your peak AC so the RMS value of your output voltage will be \$ \frac {1}{\sqrt 2}V_{DC} \$. (The RMS value of a sine is \$ \frac {1}{\sqrt 2}V_{pk} \$.)