The treatment of horizontal lists is a matter of style, not fixed rule, so there is variance. If I were editing these for a publisher, I would follow the publisher’s style. Taken in isolation, either the first or the second is acceptable handling of the list, but not the third. I would edit the third as follows because your structure requires complete sentences: There are three reasons for this. First, the ball is blue. Second, the cube is black. And third, the sphere is red. In all three examples, you ought to add “the” before “cube,” and I would call for a comma after the nouns to indicate omission, thus: There are three reasons for this: 1) the ball is blue; 2) the cube, black: and 3) the sphere, red. More often, I see horizontal lists handled with double parens, thus: There are three reasons for this: (1) the ball is blue; (2) the cube, black: and (3) the sphere, red.