Cite some examples of Monopoly, Duopoly or Triopoly? How a newcomer can get in?

In today's business world, there are some markets that are dominated by one "big bro", two or sometimes even three players which make these markets extremely hard to penetrate and compete against for newcomers. Can you give an example (or more) while, if possible, citing their common weak point(s) and how a newcomer can benefit from this and get in? Below are may tries: Monopoly: Search Engine - Google Weaknesses: Lack of privacy, SEO clickbaits Duopoly: Mobile App Market - AppStore, PlayStore Weakness: Forcing cutting 30% revenue from developers Triopoly: Cloud Providers - AWS, Azure and GCP