Perhaps it allows them to begin to open borders between each other, but keep borders to other states closed? If those three states' economies are very interlinked it might let them get closer to normality.

Legally, is this possible in the US?

In Australia for example each state has closed or is controlling their borders with each other, and this seems to largely be a decision for each state (I don't think there's a federal law either way). I could imagine some states choosing to open up while others remain closed. Or if two states made big sacrifices but got the issue under control, while the others lagged behind, those two could open borders between themselves while remaining closed outside.