classified ads section; classifieds
Refers to the section in a newspaper or magazine where short, paid advertisements are placed, often limited to a few lines.
We placed a job ad in the newspaper's classified ads section.
Refers specifically to job advertisements, while 三行広告欄 is the section where such ads (and others) appear.
Compound of 三行 (three lines), 広告 (advertisement), and 欄 (column/section), referring to the short, line-limited format of classified ads.