Every successful founder-CEO I know of is a generalist who's especially good at one of these 3, and their cofounders or early hires make up for their weaknesses in the other two areas.

That said, at large companies the dynamic is different. I know of one excellent CEO at a 5000 person company who spent almost no time on product, market, or operations. Instead, he spent all his time on the human element. Making sure that there were strong training programs, making sure that it was easy to transfer from one department to another (and instituting policies that managers had to transfer star employees every two years or less), cutting through a lot of bureaucracy to make sure that star players could get promoted commensurate to their talents, etc. The company's performance and stock price skyrocketed during his tenure, and he was considered one of the best CEOs the company ever had.