What's the difference between 礼仪，惯例，规矩？
The three wors are all nouns.


礼仪 What someone's behavior or words gives others a very good expression.for example ,the man is a gentleman

example 礼仪之邦

之：的

邦：国家（country）

礼仪之邦 ：The people in the country are all gentlemen.（indluding women and children）
惯例 the activity has already been done in this way for many years,and others' also in this way. 

惯例 mostly is used to describle activity,not behavior.
规矩 规矩 mostly is used to describle someone's words and behavior.

for example,don't make noise in public place. 

规矩 describles most people's behaviors should be done like this, not gentleman.