Sub-internships (also known as "sub-Is" and "AIs" = Acting internships) are usually for fourth years who have already done the basic third year rotation. The expectations are higher because, as implied by the name, you're supposed to act like an intern. Most sub-Is are not open to third year students who haven't finished their core rotations yet.
Clerkships may be open to third years - it depends on the program. The rotations would then be lower, because you're not expected to be quite as experienced. "Clerkships" may also be another name for "electives," and again, the expectations for electives are lower than they would be for a sub-I.