Applying the Secure Hash Algorithm 3 (SHA-3) to a file downloaded from the internet verifies the integrity of the file. SHA-3 is a cryptographic hash function that produces a fixed-length output, called a hash or a digest, from a variable-length input, such as a file. SHA-3 is designed to be resistant to various attacks, such as collision, preimage, and length extension. The integrity of a file is the property that ensures that the file has not been altered or corrupted, intentionally or unintentionally, from its original state. By applying SHA-3 to a file, the user can generate a hash value that uniquely identifies the file and its content. The user can then compare the hash value with the one provided by the source or the publisher of the file, and verify that they match. If they match, it means that the file has not been modified or tampered with, and its integrity is preserved. If they do not match, it means that the file has been changed or compromised, and its integrity is violated.
References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 3: Security Engineering, page 113. CISSP Practice Exam - FREE 20 Questions and Answers, Question 18.