You must memorize the basic anatomy of the brainstem. The right 3rd nerve nucleus innervates the right inferior oblique (IO), right inferior rectus (IR), right medial rectus (MR), and LEFT (i.e. contralateral) superior rectus (SR).
The levator palpebrae superioris (LPS) muscles on both sides are innervated by a centrally located common 3rd nerve subnucleus. This anatomy is very clinically relevant because a lesion of the 3rd nerve nucleus presents in a characteristic fashion with a classic ipsilateral 3rd nerve palsy (from involvement of the ipsilateral IR, IO, SR, MR, and LPS) in addition to a contralateral palsy of the superior rectus and levator palpebrae superioris.
Based on the anatomy, maybe you are wondering why lesions of the 3rd nerve nucleus also affect the ipsilateral superior rectus considering it is innervated by the contralateral 3rd nerve nucleus. This occurs because fibers from the superior rectus subnucleus decussate and pass adjacent to the contralateral 3rd nerve nucleus. Therefore,virtually all lesions affecting the 3rd nerve nucleus will also hit fibers from the contralateral superior rectus subnucleus as they pass by in the brainstem.
The 4th nerve nucleus travels dorsally and is the only cranial nerve to exit along the dorsal aspect of the brainstem. Before exiting, the 4th nerve fasciculus decussates so that the left superior oblique muscle is actually innervated by the right 4th nerve nucleus.
