I think it is the posterior cerebral artery.
Recall that the Posterior cerebral artery is the major blood supply for the midbrain (which houses the CN III nuceli), thalamus and the occipital lobe.
Occlusion of the interpeduncular branches of the Posterior cerebral artery will lead to weber's syndrome which affects the ipsilateral CN III nucleus and cerebral peduncle leading to ipsilateral CN III palsy, contralateral hemiparesis and contralateral lower face paralysis.
There is another syndrome i remember reading about in HY Neuroanatomy called bendickts syndrome which affects CNIII nuceli, dentothalamic tract and the medial lemniscus but i can't remember if that's also due to a vascular insult as well.
Like you've already mentioned though, a posterior communicating artery aneurysm can also lead to an ipsilateral CN III palsy. But if you are talking about blood supply, posterior cerebral artery takes the cake.