Air carrier crew reported while preparing for push back they approved the Ops Agent to pull the Jetway back from the aircraft. Air crew noticed the Jetway was pulled back and the aircraft was moving backward. The air crew applied brakes making an abrupt stop resulting in Flight Attendants being injured.

Narrative

At Gate in ZZZ; the Ops Agent had delivered the numbers early; and asked if he could pull the Jetway back. The FO (First Officer) had already finished his departure briefing; and as we only needed to complete our checklists; I gave him approval. The Jetway pulled back; and we were halfway through the Before Start Checklist; when I noticed out of my periphery the Ops Agent waving his arms at me. I noticed the aircraft moving backward; as the Ops Agent gave the X" with his crossed arms. I immediately applied brakes; and the aircraft came to an abrupt stop. With the brakes set; I immediately donned my headset; and gave a PA to the Passengers and Crew that everything was ok; and apologized about the need to abruptly apply the brakes; as we were rolling backward and had to stop the aircraft immediately.I called the Flight Attendants on cabin inter phone and asked if everyone was alright. The A-F/A said he had hit the left side of his face and ear on the bulkhead. The B-F/A had hit her arm on a cabinet. The D-F/A; I later found out; had banged his lower back against the aft bulkhead. The F/As reported the Passengers were all ok; as they were seated with seat belts fastened. I asked if the F/As needed me to call EMTs to the gate to check them over; and they assured me they were ok to continue with their duties. I next called the Ramp on flight inter phone; and stressed the need to communicate with the Pilots; before removing the chocks. Apparently; the tug and tow bar were not connected when they removed the chocks; and with the Jetway away; the aircraft rolled backward.Communicate. I have noticed the lack of communication from the Ramp to the Pilots have become endemic. From disconnecting air and electrics without communicating with the Pilots; to entering the Safety Zone before clearance; disconnecting communications before being cleared off during After Engine Start; to the chocks being removed before confirming the brakes are set and tow bar connected; the communication has been getting horrible. The risk of someone getting seriously injured or killed is a real threat if communication with the ramp and flight deck isn't improved through training."