What is the horsepower of a 3 liter engine?

There's no single answer to the horsepower of a 3-liter engine. Horsepower is determined by many factors *beyond* engine displacement (size, in this case 3 liters). These factors include:

* Engine design: Naturally aspirated, turbocharged, supercharged, etc. Forced induction (turbo/supercharger) significantly increases horsepower.

* Specific engine technology: Valvetrain design, fuel injection type, compression ratio, etc., all play a role.

* Tuning: How the engine is tuned and calibrated affects its output.

A 3-liter naturally aspirated engine might produce anywhere from 150 to 300 horsepower, while a turbocharged 3-liter engine could easily exceed 400 horsepower, or even much more in high-performance applications.