pick up 和 buy 和 get 有什么区别？如果难以说明的话，请教我一下例句。

Usually it would mean this:
To pick up:
I will pick up my sister from school.
To buy:
I will buy cake at the bakery.
To get:
I will get the book from my bag

However in normal conversations, many people use to pick up, to buy, and to get to mean to buy

Examples:
I will pick up some orange juice at the store. (It is implied that the person will buy it)
I will get some orange juice at the store (It is implied that the person will buy it)
I will buy some orange juice at the store