Are the internal drives on a MacPro3,1 hot swappable?
In 12 years of owning Mac Pros, I've never tried this.

MacRumors, however, says a resounding "No". Don't do it.

Apparently the protocol supports it but the hardware must too. XServes supported it, Pros don't.

Relevant threads…
Can you Insert Drive While Running Mac Pro ??
How to rescan SATA ports without restarting?

I also found on ArsTechnica - Did I fry my Mac Pro?