It splits fairly naturally, except for IL (an E. coast state in the midwest) and perhaps MN. All 3 have ports. Most of the oil is in Middlesex, but that's Ok because the Essex and Wessex governments don't like fossil fuels, anyway.
Some of the states near the borders may want to either hold a plebiscite or split (e.g. VA into NVA and SVA), so most citizens get their wish. E. MD would not be happy in Essex but could join up with SVA in Middlesex. E. Washington and Oregon may also want to join up with Middlesex.
All 3 new countries could do what they like to do: Essex to have high taxes, strong unions, and take in lots of welfare recipients, Wessex take as many of the illegals as they want and practice LGBT to their heart's content, Middlesex could continue with their old value system, hunt, and put up a fence along the Mex border. The armed forces would be split equally between the 3 new countries who could then fund their pieces as their citizens prefer.
The blacks would be roughly equally split between Middlesex and Essex if they stayed in situ but could and would move to another country as they desire, as could the Amerinds.
Wonder if that split could be vitrual, and not necessarily geographic.