Talk:On Your Marks/@comment-11634302-20160514073144/@comment-4531340-20160514082826

Probably what this episode indicates - if an episode is focused on one of them specifically, they'll try out new things and discover a pony with a cutie mark problem (and that CMC can either get together with the other two to help solve the problem, solve it on their own, or some combination therein). When it's not, they can be used in all sorts of roles (similar to the Mane Six, now that I think of it) - just look at how Scootaloo is used in Newbie Dash.