1、assure有“使确信，向……保证”的含义，表示为了鼓舞士气或者使某人对自己的信仰和想法充满信心，例如：
She assured me that she would not be late.
她向我保证决不迟到。
Don't worry about your difficulties in Chinese. I can assure you mine are still greater.
不要担心你在中文上的困难。我可以向你保证，我的困难更大。
2、ensure 有“保证，担保，确保”的含义，相当于insure或者guarantee，例如：
Supervisors should make every effort to ensure the safety of their workers.
管理人员要尽全力保证工人的安全。
Anny was to ensure that the computer was packed safely.
安妮要确保电脑包装安全。
3、insure 有“投保，给……保险，承包”的含义，通常指钱，侧重金融经济方面更常用insure。例如：
The house is insured for two million dollars.
这个房子投保了200万美元。
to insure one's house against fire
给房屋保火险