The laws for those three electoral votes are made by Congress (in the same way that state electors are controlled by the state legislatures) and the bill that passed the House last year (and presumably is the basis for any efforts this year) would simply repeal any provision for their election. It would also provide for expedited consideration of a constitutional amendment repealing the 23rd Amendment, but until and unless that happened those three electors would simply not be elected. The remaining capital district would have no legal residents, with anyone physically residing in its boundaries being assigned to their last state.