At this point, with the lines semi stabilized, both sides really want to think about how much treasure and lives those three Oblast's are worth.
Russia would need to give some real concessions (Like the rest of Ukraine in NATO) but they would get their land bridge to Crimea, and Putin could claim a victory at home, hopefully forestalling the normal murderous thrashing of a failing dictator.
Never mind US and EU riches, which are significant, tens or hundreds of thousand of people could die grinding away at each other over fortified defenses if we (or Putin) decides to drag this out to the bitter end.