Assuming all three units are the same ...

1. Create a new asset using the original "placed in service" date with 1/3 of the Basis and 1/3 of the Prior Depreciation.
2. Create a new asset using the original "placed in service" date with 2/3 of the Basis and 2/3 of the Prior Depreciation.
3. Delete old asset.
4. Enter sales information in the new asset that has the 1/3 of the Basis and Prior Depreciation.