There are occasions when using more than two video cards that Windows and the Nvidia driver properly register all cards, but OctaneRender does not see them. This can be addressed by updating the registry. This involves adjusting critical OS files, it is not supported by the OctaneRender team.
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E968-E325-11CE-BFC1-08002BE10318}
DisplayLessPolicy
and LimitVideoPresentSources
and set each value to 1. OctaneRender should now see all video cards.