I have no experience with ePub, but from looking at the wiki, the CSS looks to be:
font-family : "Embedded_font_name";
font-style : normal;
font-weight : normal;
src : url("path/Embedded_font_name.ttf");
Calendas Regular is what you're looking for, so that would probably translate as "font-style : regular;".
But yeah, it's kinda messed up, having 3 separate entries for the same font. Just change the Family Name (and nothing else, don't bother) from "Calendas Bold" and "Calendas Regular" to just "Calendas". That should take care of it.