For a while now, five big players — Anthem, Humana, UnitedHealth Group, Aetna, and Cigna — have dominated the U.S. health insurance market. But it looks like that number will soon drop to three: A few weeks ago, Aetna inked a deal to buy Humana, and this week saw the final touches put on a merger between Anthem and Cigna. The latter deal isn't finished yet, and may face scrutiny from antitrust regulators.