Seems to be indeed a bug.
When I use the equation editor to add it, it comes out as "LaTex: 3^{\doubleprime}" when I press "Done", even though in the editor itself it shows appropriately.
Adding \prime twice does work, so that should be a workaround until (if) \doubleprime is fixed.
It does't seem to be listed as a known issue, so a bug report may be appropriate.
--EDIT--
\doubleprime doesn't seem to be standard in MathJax, which is what Canvas uses to represent LaTex for viewers, but it is implemented in MathLive (see their github here), which seems to be what Canvas uses to edit LaTex in the RCE.
That's probably why it works in the editor, but not when viewing.
