Third NSW man charged for serious assault

A third man charged over the assault that left a Sydney teen with brain injuries also allegedly impersonated a police officer.

The 42-year-old was arrested on Friday morning in relation to the assault on an 18-year-old at North Richmond, in the city's northwest outskirts, on February 22.

He was charged with multiple offences, including impersonating a police officer and causing grievous bodily harm with intent.

It follows the arrest of two 20-year-old men on Thursday.

The victim has been in Westmead Hospital since the incident and it's expected he will have long-term brain injuries, police say.

The three men were set to appear at Penrith Local Court on Friday.