Thanks on clarifying and as per
Forum Charter [webmasterworld.com], please do not use your real products, use wigets, or if additional clarification required, use generalised example products different to yours.
Site B is a copy of Site A so all the URLs are the same and the code.
Which site ranks, A or B? Is there canonical link element (rel=canonical) on URLs of one site pointing to another?
Site C is already well founded and sells a product tote that is sold on Site A. <for example pens>
Similar question - if you search for your <pencils>, which site ranks, A or C? Is there canonical link element (rel=canonical) between pages of these two sites?
Since all three sites already exist, you could choose a handful of pages which currently rank on site A and redirect them to site B and site C and see what happens with ranking of these pages. This can be your initial test.
But all together I would do the other way around to what you were asking under points 1 and 2 of your opening mail.
I would firstly redirect all <pencils> pages of site A
and site B to site C on a relevant page-to-page basis. If there is a page that exists on sites A/B and is not on site C, it should be created on site C and pages from sites A/B redirected there. If site C has rel=canonical to A or B, these need to be removed.
This would result in having site A (and its copy site B) focusing on <pencils> only and all <pens> content on site C. Wherever possible, change internal links for <pens> that point to site A/B onto equivalent page on site C.
I would then let Google digest this - depending on the site size you may look here between 1-6 months for Google to fully process this.
Once this is done, then you can redirect the remainder of site A to site B, and execute Domain Move in WMT since by that time all <pens> content will already be on site C. If you are using rel=canonical, be careful that they still do not point to site A after redirects and site move.
Also make sure that once you start your site split, none of sites are blocked by robots.txt or meta robots noindex.