Taking an artwork into a new medium such as 3D animation and/or creating fan fiction based on an existing artwork usually does not fall under Fair Use. For more on fair use, please see this Wikipedia entry: http://en.wikipedia.org/wiki/Fair_use
Such works are generally considered "derivative works", and the right to create such works is one of the exclusive rights of a copyright holder. 17 USC § 106(2)
If someone creates a derivative work without authorization from the copyright holder, they can be liable for copyright infringement. Whether we will seek to go after them or not, is another story... If the purpose is educational or demonstrative and it's limited to a rather short piece, it may indeed be Fair Use. But if someone starts making a new series or starts making 3D versions of several existing episodes, it's probably infringement - especially if they're taking "donations."