no tethered support for canon mk III cameras :-(
From the release note PDF
http://216.93.185.225/CO_WIN/bc67d824-7308...378%20_win_.pdfTethered camera support
Capture One PRO has tethered support for the following cameras:
Phase One:
P 45+, P 30+, P 21+, P 25+, P 20+
P 45, P 30, P 25, P 21, P 20, H 25, H 20, H 101, H 10, H 5, LightPhase
Canon:
1D Mark II N, 1Ds Mark II, 1D Mark II, 1Ds, 1D
5D, 20D, 10D, 350D/Rebel XT, 300D/Rebel, D60, D30