警告！今日親身經歷本人被屈錢黨屈錢！呼籲各位人兄大陸坐大巴要小心！

I had similar experience in a bus in Guangzhou 3 years ago, but just with two guys.  The three guys actually were inside the same bus with you, but you just noticed the goods carrying man.  The thinking that was it really my fault in breaking his glass reduced my ressistance.  

If it happens again, I still not sure what to do.  Maybe say that willing to pay for any compensation of loss ( loss of goods, loss of job, whatever they sufferred, even $100K), but must call for GongAn to resolve.  If they refuse to call GongAn, it is certainly a robbery. Then the only solution is still threat them of GongAn.  What can be sure is that they don't dare to rob or hurt you, you they dare, they have no need to play the dramma.