You should duplicate the description of A in each of the others. This ensures that every patent application has a complete description. If your invention relies on A, but you don't describe A (and it's not something common in the art), then you would not be enabling your invention. If you have not enabled your invention in a provisional application/priority document, there is an argument that you would not have a valid priority claim. Better to avoid this.
Moreover, while the US allows you to incorporate another document by reference (that is, you simply refer to it in the description), this technique does not fly in other countries (notably Europe). So if you want international protection, you should duplicate the description.
As an alternative however, you can file a single patent application which describes (and at the non-provisional/Convention stage, claims) all the inventions. You will receive a restriction requirement (in the US) or a unity objection (elsewhere) when you file a non-provisional/foreign application. You would then have to narrow your claims to one of the inventions, and file divisional applications to the others. This defers the costs of filing all the applications by a couple of years.