LTspice three phase rectifier simulation
It's important to note that a rectifier's positive and negative outputs are the highest and lowest voltages from the input respectively.  That mean that the negative output isn't ground.  By grounding the negative output in the right circuit, you short any input that has a negative voltage.
You can still use the circuit on the left but you need to set the negative output as some sort of virtual ground.  Connecting the two grounds will cause a short circuit, and the two grounds will oscillate with respect to each other.  The easiest thing to do is ignore the ground from the generator, so you treat it as a delta source rather than wye.