Let there be 3-phase rectifier connected to the standard electricity grid with 400V between phases. Measuring voltage between one of the phases (AC) and negative terminal of the rectifier output (DC), what range would we get? Would the voltage stay always positive?
Your output will be negative assuming the diodes are not ideal (ie. they have some forward voltage drop --> Vf > 0). If the phase being measured is the lowest voltage, then it becomes the DC negative terminal (that phase's bottom diode will be on), so the measurement will be -Vf. Otherwise there is a phase with a lower voltage and the output will be the difference of the phase being measured and the phase with the lowest voltage. The highest this value could be is about 1.732 (sqrt (3)) times your input voltage minus Vf.
Simulation results with Vf = 1 V:
Note you can't really see the -1 V output, but it is there.