One straphanger was robbed at gunpoint and two others socked in the face early Sunday morning in another round of public transit violence, according to law enforcement sources.

The mayhem began at 5:30 a.m. on a northbound 6 train in Manhattan when three men assaulted and robbed a 41-year-old rider, sources said.

One of the assailants asked the man, “Why you lookin’ at me?” then another punched him in the face with a closed fist, sources said. The three men fled with the victim’s phone.

Three hours later, at around 8:30 a.m., a customer heading to work on a Brooklyn-bound F train was robbed at gunpoint by three men, who fled with his cellphone and cash, according to sources.

The man reported the crime — which occurred in the tunnel between Manhattan and Brooklyn — when he got to work.

In a third incident at 9:36 a.m., a 56-year-old woman allegedly socked a 70-year-old who was waiting on the northbound platform for the Lexington Avenue line at Union Square.

The victim suffered bruises, swelling and a cut to her face but refused medical attention, cops said. Her alleged assailant, who sources identified as Tammy Long, was taken into custody without further incident.

The attacks on Sunday morning come on the heels of a string of brutal slashings two days ago.

Transit officials have repeatedly asked NYPD for more police patrols on the subway.