Sorry, it's considered a cheat. If someone else tells you, that's fine. But just be known, you could be booted from a server if this is found in your configuration.
Using 3rd person in MP is considered a cheat. If the server has cheats 0 the 3rd person toggle is disabled. Besides that, it gives you greater peripheral vision and a height advantage. You can see around corners and over walls/objects that otherwise you couldn't. Doesn't it mess up the accuracy of the crosshair anyway?