Flight Crew Last 90 Days 30; Flight Crew Type 11700
Situational Awareness
Conflict Ground Conflict; Less Severe; Ground Incursion Taxiway
Flight Crew Took Evasive Action
Airport; Human Factors
Three separate events of vehicles ignoring our aircraft movement caused me to have to abruptly stop our attempt to begin taxiing. Our flight attendants were almost knocked off their feet during our stops.  One catering truck would have collided with our wing. One mechanic towing an O2 cart would have hit the right engine; and a tug baggage cart came dangerously close [to] the operating right engine.
A B757 Captain reported three separate occasions of having to stop during taxi out at ORD; to avoid vehicles that failed to yield to his aircraft.