Hi Lee and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, the EJECT button is for a CF card and not the SD card. For it to engage in the out position, a CF card must be inserted first.
On the otherhand, the SD card does not have an eject button on a Canon EOS 5D Mark III. The SD card can be released if pressed on the SD card edge.