What are the three largest real estate companies in the US?

The three largest real estate companies in the US are Realogy Holdings Corp, Keller Williams Realty, and RE/MAX Holdings Inc. Realogy Holdings Corp is the largest by revenue and owns several well-known brands such as Coldwell Banker and Century 21. Keller Williams Realty is the largest by agent count and operates as a franchise with a focus on agent training and technology. RE/MAX Holdings Inc is known for its agent-centric business model and global presence.