Can I mirror my second and third monitors but not the first for a presentation?
I don't know how to do it exclusively in Keynote, but it is indeed possible to mirror two of three displays.

Open Display preferences, click the ? at the lower-right of the "Arrangement" tab of "Displays" system preference. To set up a combination of displays showing mirrored and extended desktop images, set up the three displays as an extended desktop, and then Option-drag a display icon onto another display icon to mirror the two displays.

Instructions with pictures.