Canon EOS 5D Mark III Instruction Manual page 190

D Flash Photography
If the camera is used with a flash unit or flash accessory dedicated to

another camera brand, the camera may not operate properly and
malfunction may result.
Do not connect to the camera's PC terminal any flash unit requiring

250 V or more.
Do not attach a high-voltage flash unit on the camera's hot shoe. It may

not fire.
A flash unit attached to the camera's hot shoe and a flash unit connected to
the PC terminal can both be used at the same time.
190