Thank you for your excellent response, Alan.

Reviewing my post, I did not explain the situation 100% accurately.

Rather than integrating all our shops, in fact we would just be taking one of the shops we run, our main site, and wondering whether to consolidate with two new shops running on two new domains (these domains being excellent keywords).

An important point is that we changed the name of the site and domain name mid-2011, to a more international name. At first this clearly had an effect on our Google ranking, although having just had the best Christmas sales ever, we feel this has succeeded. (301 redirects were set up)

It would appear that the decision whether or not to integrate now to a unified domain would be affected by this fact. The name of our current main site would not be suitable as an umbrella term for all.