In general, a typical silicon rectifier junction will have a MINIMUM PIV (peak inverse voltage) rating of 50V.
But while this sounds like it would be sufficient, just remember that a 48V nominal system sees voltages well in excess of 50V.
A good rule of thumb is AT LEAST TWICE what you intend to ever see, which helps with reliability. 100V PIV rated diodes would be the lowest I would personally go for a 48V system, but that's just me. Others may have success with closer margins.
A diode won't necessarily catastrophically fail when it's PIV is reached; this would depend on the current that tries to flow in the process. It's still not good practice, because at the very least, the diode is in danger of conducting "backwards" while it's PIV rating is being exceeded, defeating the purpose of having the diode there to start with. Enough current flows during the reverse conduction, and POP! And in my experience, the current capacity when a diode is in avalanche mode (reverse) is nowhere NEAR it's forward rating.
Doesn't do you much good to go too far above the "double" rating, however, either. Just increases cost, and can mean more heat (as the typical Vf (forward conduction voltage drop) is likely to be higher as well, resulting in more power dissipation at the junctions.
I can't say for a 3 phase bridge, but for single phase, calling it closer (ie 50V PIV on a 48V nominal system) might not be an issue, since there are effectively two diodes "in series" all of the time, inherently raising your margins by default.
Others could probably comment on that and clarify a little better than I.
Steve