Ah, I see ... I didn't realize that you wanted to do a partial consolidation only. That way You will still have maintenance of three sites, not one.
Personally I would opt for consolidation of all pages on one domain.
If you do option one, you will essentially only transfer part of site B & C to site A.
The remaining parts of site B&C will lose the link juice from the pages you redirect to domain A (as those pages will no longer link back internally in terms of on-site navigation). So, those two sites will become smaller, and less powerful. The counter-weight of this is one site that does not grow in content (as all products were there anyway) - it only gets two extra links per product page (and some second-order "inherited" link juice with those). So, it's a marginal gain in link juice.
However, if you consolidate all pages on one domain, that domain will become significantly stronger, both in terms of content and in terms of link juice.
So, if "total cosolidation" is option 2, I would say that option 2 is best.
Beware, though: Regardless of what you do, it may take a while for the full effect to show in the SERPS as soon as you start doing redirects on a larger scale. SOme rankings/keywords may even tank for a while, until Google decides that the move is "legit". I'm sure you already realize this, but I'm saying it just to be on the safe side.