Hi Wrench,
Is it a single website or 3 seperate websites before the seperation? If you have multiple websites then you can redirect the websites so that all the link juice will be passed to the new domains. If it is a single website and if the 3 entities are seperated by folder structures. Then you can do a page by page 310 permanent redirect of the corresponding folders to the new websites if the 1st website owner allows you to do that. Then all the SEO goodies of the particular sections of the pages only will be passed to the new domains.