I wrote this about beer, but the idea is the same...
The thing to remember though is that if you are smelling this during fermentation not to worry. During fermentation all manner of stinky stuff is given off (ask lager brewers about rotten egg/sulphur smells, or Apfelwein makers about "rhino farts,") like we often say, fermentation is often ugly AND stinky and PERFECTLY NORMAL.
It's really only down the line, AFTER the beer has been fermented (and often after it has bottle conditioned even,) when you should consider using this or the off flavor charts to diagnose the beer.
I think too many new brewers focus to much on this stuff too early in the beer's journey. And they panic unnecessarily.
A lot of the stuff you smell/taste initially more than likely ends up disappearing either during a long primary/primary & secondary combo, Diacetyl rests and even during bottle conditioning.
If I find a flavor/smell, I usually wait til it's been in the bottle 6 weeks before I try to "diagnose" what went wrong, that way I am sure the beer has passed any window of greenness.
Fementation is often ugly, smelly and crappy tasting in the beginning and perfectly normal. The various conditioning phases, be it long primary, secondarying, D-rests, bottle conditioning, AND LAGERING, are all part of the process where the yeast, and co2 correct a lot of the normal production of the byproducts of fermentation.
Lagering is a prime example of this. Lager yeast are prone to the production of a lot of byproducts, the most familiar one is sulphur compounds (rhino farts) but in the dark cold of the lagering process, which is at the minimum of a month (I think many homebrewers don't lager long enough) the yeast slowly consumes all those compounds which results in extremely clean tasting beers if done skillfully.
Ales have their own version of this, but it's all the same.
If you are sampling your beer before you have passed a 'window of greeness" which my experience is about 3-6 weeks in the bottle, then you are more than likely just experiencing an "off flavor" due to the presence of those byproducts (that's what we mean when we say the beer is "green" it's still young and unconditioned.) but once the process is done, over 90% of the time the flavors/smells are gone.
Of the remaining 10%, half of those may still be salvageable through the long time storage that I mention in the Never dump your beer!!! Patience IS a virtue!!! Time heals all things, even beer:
And the remaining 50% of the last 10% are where these tables and lists come into play. To understand what you did wrong, so you can avoid it in the future.
Long story short....I betcha that smell will be long gone when the cider is carbed and conditioned.
3 weeks is way to fast to be bottling especially if it smells like sulfur. My guess is your yeast needed some nutrients or wasn't stirred enough in primary. 
