The CC BY-SA 3.0 license applies to the publication as a whole, but if it contains materials or content from a third party, then those materials/contents still keep the original license, and they need to be marked accordingly. As specified in Creative Commons:
Marking best practices also apply for any third party content your work incorporates. Third party content refers to material created by others, or more precisely, in which the licensor is not the copyright holder. Third party content could be offered under a Creative Commons license, restricted by All Rights Reserved copyright, or anything in between. You should obtain any permissions required for your use of third party content and abide by any license restrictions.
Using third party content in your work that is not offered under the same license terms as the rest of your work may require additional marking. If you include works offered under other Creative Commons licenses, additional marking may be required for attribution. If you include third party content in your work that may not be available for reuse under the same terms as the rest of the work, you should warn users and mark it with any additional information that may be helpful. CC offers additional explanation and tips on giving thorough notices and marking for works.
Even if the Stack Exchange post is CC BY-SA 3.0, if you specify that some parts are not covered by that license, then they are not, and they will keep their original license.
Which brings us to the question on hand: may I edit a question with a JSFiddle link to incorporate the code? It will depend on the specified license on the JSFiddle.
So, unless the JSFiddle specifies a license that is incompatible with the CC BY-SA 3.0 license, you should be able to copy the code from JSFiddle into the question itself, but you will have to:
- Credit the author and origin of the code (link to the original JSFiddle)
- Specify that the code may not be available for reuse depending on its license.