The difference between going to the library once every three days and going to the library once every three days is that the total number of days is different, the intervals are different, and the times are different.

First, the total number of days is different.

1. Go to the library once every three days: The total number of days to go to the library once every three days is four days.

Go to the library every three days: the total number of days to go to the library every three days is three days.

Second, the interval days are different.

1. Go to the library once every three days: The interval between going to the library once every three days is three days.

2. Go to the library every three days: The interval between going to the library every three days is two days.

Third, the number of times is different

1. Go to the library once every three days: At the same time, the frequency of going to the library once every three days is less than three days.

2. Go to the library every three days: At the same time, go to the library every three days more often than every three days.