[jQuery] jQuery validate, need to require subset of fields with same name

This is regarding use of jquery validate plug-in.
I have ten fields of type="file" with name="thumbs[]"
I have added class="required" to the first three, but does not give
the intended result. obviously i'm trying to require the first three
of the ten possible thumbnails image uploads.
how do i accomplish this? does jquery validate work with #id
selectors?
Many thanks
Jason