Three people have been arrested over what police have described as the "systematic and ongoing physical abuse" of a four-year-old boy in Sydney's west.

The boy was taken to The Children's Hospital in Westmead in May this year suffering from a raft of serious injuries, including brain haemorrhages, multiple fractures, a broken tooth, and severe bruising to his body.

The boy is now recovering from his injuries, police said.

Detectives from the Penrith child abuse squad launched an investigation into the child's abuse, and on Tuesday arrested a man and two women at a home in Doonside.

Police said one of the women, aged 36, was taken to Mount Druitt police station where she was charged with assault occasioning grievous bodily harm (reckless) and concealing a serious offence.

The other woman, aged 28, and the man, aged 26, were taken to Blacktown police station. Both were charged with assault occasioning grievous bodily harm (reckless) and concealing a serious offence.

All three have been refused bail and are due to appear in court on Wednesday.

"In court, police will allege the four-year-old boy was subjected to ongoing and systematic physical abuse while in the care of the three adults at a home in Doonside," police said in a statement.