JohnF. If you are referring to the term as borrowed from Japanese, then the 3rd line can be (and often is) used for crawling. For example, the eminent Hayashi Yutaka (editor of Kido among other things) says: 2線や3線を低くハウ手. Admittedly he then goes on to say, "However, on the 3rd line there are also cases where it is appropriate to call it a 'push' oshi, while on the 2nd line the term oyogu swim? has a slightly different nuance."