Physics says there will always be three pieces.
Can you cite a reference for that claim?
if one removes the word "always" then the answer lies in:
For flywheel shaped disks (High speed turbines) the stress field between radial stress and tangential stress is relatively equal over much of the disk (not the OD or ID) and at
overstress conditions when the disk fractures the maximiun energy state of each piece released (in translation and rotation) occurs at 127 degrees of arc of the pie shaped released sections.
Wont happen if the disk fractures from a gross single defect condition at normal speeds (expect one very large, one moderate and several small)