This is the 50th most frequent Chinese word.
三
"Three."
In this sentence, '三' functions as a numeral, indicating the quantity of books.
我有三本书。
I have three books.
Here, '三' is again a numeral, specifying the number of sides belonging to a triangle.
三角形有三个边。
A triangle has three sides.
In this case, '三' is used as a numeral to describe the duration of time in days.