There are 420 books in a yard.

(2) If only one 3-layer bookshelf is used, the average number of books on each bookshelf is:

420÷3= 140 (Ben);

Answer: On average, there are 140 books on each shelf.

(3) In order to use fewer bookshelves as far as possible, only three bookshelves are used, 140 storybooks are put on one floor, and then the remaining 20 books and 120 comic books are put on one floor; Putting 90 science and technology books and 50 English books on the last floor can ensure that there are 140 books on each floor, and at the same time, it does not violate the principle of "putting the same number of books on each floor, putting only two books on each floor at most, and using as few bookshelves as possible".