Volume 3 typically picks up where the fundamentals of mechanics and electromagnetism begin to settle, pushing forward into the complex interactions of the physical world. In many editions, this volume is dedicated almost entirely to , Optics , and Modern Physics .
You are taking a junior-level modern physics course, your professor specifically assigned Ohanian (rare but happens), and you cannot afford the $150 used hardcover. In this case, use your library’s interlibrary loan to scan the specific chapters.