Numeric and numerical are strictly adjectives, numeral can be both noun or adjective but Americans would likely regard numeral as predominantly being a noun.
I'm not sure why, but of all your choices, I think "focus on numbers" sounds the clearest and best. When I hear numeric or numerical, I usually think of it as something represented by numbers, like numerical equations or numeric symbols. This is probably just me, but "focus on numbers" and "numerical focus" don't sound like the same thing.
(not a teacher, just a language lover)