三人行 = three people walking together
必有我师 = there must be a person who can become my teacher
三个人走在一起，其中一定有一个人可以作为我的老师。这个谚语是指应该不耻下问，虚心向别人学习。无论是谁，他 / 她一定有优点。人本来就不一样，三个人走在一起，你会发现别人的优点，进而向别人学习。这个谚语也教我们，不要看不起别人。
三人行，必有我师 literally means three people walking together, there must be a person who can become my teacher. It also means we must be humble and learn from other people. Everyone is different and has something that we can learn from him / her. This idiom also teaches us not to look down on someone.
In love relationship, a good lover teaches you the sweetness of love while a bad lover teaches you the bitterness of love. We need to learn both aspects. If you only know the good things in life, you may not appreciate what you have. Open your eyes and heart to learn, even though the relationship does not end happily.
In in-laws relationship, instead of disliking your in-laws, find the virtue in them. One day, you may find out that they are actually better people than what you thought they were.