I'm no expert either, but my understanding is that it is in fact copyrighted and you need permission (read as pay) for use and whether or not you can profit from such usage (the company will want a piece of that cake too). For 3d characters resembling real people, I think the likeness of a person is also protected. However, a company may choose not do go after people using their brand if the usage does not affect them in a negative way. In fact some companies may see it as free promotion. But is always better to play it safe and don't take the risk.

Example.

Say a person creates a 3d render (or any art really) of a happy family having fun at the beach and one of the kids happen to be drinking coca cola. However Coca Cola is not even the focus of the render, might as well could have been Pepsi or whatever. The image is a very well done render and sets a nice tone family oriented and it becomes quite popular, including its creator. The person keep getting famous with more creations and keeps the gimmick of using coca cola every now and then in their renders. All happy positive art. Coca Cola may choose to remain silent and don't do anything about it, because is free promotion and does no harm to the company. 

Well, it turns out the creator was accused of *insert crime here* and is found guilty. Now Coca Cola could see this as a negative impact for their brand and may want to protect their image and disassociate themselves from this person and can sue the person for unauthorised use of their logo.

That's just one of example of an infinite possibilities.