i just toss mine in the backpack i thought this is one of the reasons they are build with that magnesium alloy body. When i had the 550d then i was more careful that thing felt very weak all plastic.
Agree -- the 5D3 body
will 95% likely be fine, but there are other features to consider. I've had a lot of lens caps fall off when I just throw it in a bag with other contents, once resulting in a scratched filter (thankfully not the lens' front element). Also, multipurpose bags like backpacks and messenger bags often carry 'other' cargo that can leave junk, crumbs, dirt, dust, etc. which can scratch an LCD or do a number on the grippy bits.
For me, I don't like loose items clawing at any part of the camera when it's in a bag, so I usually use the previously linked Tenba photo insert. I'm not arguing my way is better -- it's entirely personal preference.
However, when I'm in a hurry or if I need a mixed payload of camera + 'other' in the bag (clothes, work items, pens, etc.), I sometimes just wrap the 5D3 + lens with a winter hat and call it good. I've had zero issues with that thus far, but I do admit I baby the bag a bit more in that arrangement.