Hi,
One way to do this is to decide which site is going to be the main site (site A) sat on C and copy this data over. If you are bringing in site B, then this can sit at another structure level - you will end up with this...
Site A --> Site C, main pages
Site B --> Site C, product pages
That then brings in everything from both sites to the new domain.
You then want to redirect both of the old sites to the new one, but don't redirect everything to the root. That isn't a good use of 301 mapping. You need to be mapping on a page level so that you will see...
www.sitea.com/about us -301- www.sitec.com/aboutus
www.siteb.com/newproducts/hammers -301- www.sitec.com/newproducts/hammers
There can be differences in the URL's - you don't need to stick with the same structure as the other sites if it doesn't make sense, but always map pages to something very similar.
Page level is the only way to go if you want to maintain a seamless transition for users as well.
Also, don't expect to hit a switch and do this all at once. You can do this over a period of time because to the user, they will just be redirected to the new pages. You will retain more link juice like this.
This is quite a lengthy process and I am sure I have missed the in-between bits, but this is the basis of what you want to be doing.
Others might chip in with other suggestions for you.
-Andy