As for the actual process, it would be fairly easy... simply import your MultiBit HD seed into Electrum, move your BTC to a new wallet... then import your MultiBit HD seed into Electron Cash... move the BCH to a new BCH wallet... finally, you would need to either import your seed into Coinomi to get BTG... or use
https://iancoleman.io/bip39/ along with MultiBitHD seed to get the private keys to import into BTG Core.
So, if I import my seed into Electrum but it reports an empty wallet when my Multibit says I have a few mBTC left it is the wrong key?
I also have Multibit classic still installed, so i tried exporting the private keys from there. When I import those in a new Electrum wallet my balance is also different from Multibit HD. I guess there must be some sort of private key only in Multibit HD, but I understand I can't get that out without the seed?