A family of Microsoft word processing software products for creating web, email, and print documents.
I do not think that there is anything on Graham's site or in my Merge Tools - 20130627 Add-in that can be downloaded from
https://skydrive.live.com/?cid=5AEDCB43615E886B&id=5AEDCB43615E886B%21566
that will do precisely what you are after in one go. However, I believe that it would be possible to achieve what you are after by splittin your document into the A3 forms and the covering letter and first merge the A3 forms to create documents which you would then use as attachments with the merge with attachments facility.
Then you would modify the data source so that it contained a field or fields that contained the path and filenames of the forms to be attached to the covering letter, the merge of which would be performed by the Merge with Attachments facility.
Alternatively, for a fee, I could create a customized version for you that would do the whole thing in one go.