No, we don't.
We have 37 questions tagged with any of those tags. Of those 37, we have
- 26 questions tagged with just mass-effect
- 1 question tagged with just mass-effect-2
- 4 questions tagged with just mass-effect-3
This leads me to the conclusion that these tags add very little over the general tag.
They should be separate tags. They're each for different works that will each have their own questions. The misuse of mass-effect as a franchise tag should end though, as that confuses the first game with the franchise. I propose a mass-effect-franchise for this purpose.
Synonymizing them into one would be like merging all the individual Star Wars film tags into one. It destroys the correctly fine-grained nature of the tags.