> Is this sentence correct?

> > 1. **Although** I would like to start, **nevertheless** it is not possible.

> Or should it be:

> > 2. **Although** I would like to start, it is not possible.
> > 3. I would like to start, **nevertheless** it is not possible.

Your example #1 is fine. 

Consider this example from the 2002 CGEL, page 776, example [2.iii ], which is structured very similar to your example:

> [2.iii ] *Although he affects a gruff exterior in many instances, **nevertheless** he is fundamentally a man of warm heart and gentle disposition.*

and their explanation:

> Another very common case is represented in [2.iii ], where the connective adjunct has a **reduplicative** role: the relation between the main and subordinate clauses is already marked by *although*, so that *nevertheless* simply marks this relationship a second time.

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ASIDE: As to your other two examples, example #2 is fine; but example #3 might be a bit controversial in that some people might see it as being a *comma splice*, and so they might expect or demand a semicolon or period to be used instead of a comma. Example #3 is okay to me for informal style, but if I'm writing in a very formal style or for a formal register then I might think twice before using a comma there like that.

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NOTE: The 2002 CGEL is the 2002 reference grammar by Huddleston and Pullum (et al.), *The Cambridge Grammar of the English Language.*