.'s really help BTW... (I'm not trying to be an a-hole. No punctuation makes it much harder to understand.)

3.0L is the size of the engine's displacement. 3 liters of air is pulled into the combustion chamber every 2 turns of the engine. The higher the liters, the more power the engine can put out.

Example:
2.4L (240D): 68HP
3.0L (non-turbo): 90HP
3.0L (turbo): 120HP

The turbo forces more air into the engine. It makes a 3.0L have an output like a 4.0L engine without the larger size.