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We don't know how large the chapters are. They could be 1 page long or 100 pages long (plus we don't know how many pages the book is either).
In this statement we are told that the first three chapters are larger than the other 12. This doesn't tell us anything about how much she has read though.
S1 & S2.
It is possible that she had read at least 1/3 of the book if the first 3 chapters are sufficiently large. However, it still might not be enough.
Let us assume the book is 150 pages and that the first 3 chapters are 15 pages. We'll also assume the 4th and 5th chapters are 1 page long and chapters 6 to 15 are all less than 15 and share the remaining pages.
Has she read at least 1/3 of the book (which is 50 pages in this example)? No, she has only read 47 pages, which is not enough. If you adjust numbers a bit, you will see that it is possible for her to have read >1/3 of the book.