دَلْوٌ — nominative, ḍamma on the final wāw: a bucket.
دَلْوًا — accusative, fatḥa on the final wāw: a bucket as an object.
دَلْوٍ — genitive, kasra on the final wāw: a bucket after a genitive governor.
Why? In دَلْو, the lām before the final wāw bears sukūn. The wāw carries visible case vowels. Unlike the earlier example مُوسَىٰ, its three cases are not all estimated.