Not sure if "market power" is strictly synonymous with "market share" though. It is probably possible to argue they have much more power than the 3% would suggest.

You could even argue that "power" in this context is actually framed around an idea of latent market share - the amount of market share that can be achieved outside of normal competitive means.

But yes, in general this would underscore the criticism of US competition policy that it's designed to close the gate after the horse already bolted.

I could see that. However the only flaw with that graph is that the scope is world-wide. I believe the Sherman Antitrust Act cares primarily about US operations.