I like all three contenders. But I think I might choose
Margaret too, partially because it isn't as popular as the others. (Don't have time to check but I'm pretty sure).
No I don't think
Margaret Catherine flows terribly well, but it's not bad. But really I don't think the flow matters much, unless she goes by both names. Otherwise the mn is usually unnoticed anyway.
Catherine Margaret, but going by
Margaret, seems unnecessarily complicated. Might as well give her the fn
Margaret, seems to me.