My oops post above used Calibre would be easiest
otherwise: you could unpack the additional books and add the files to the first.
WARNING:Sigil does not support Multiple (Text) folders. All filenames must be unique or bad things happen
This is really simple if both books use the same stylesheet.
If they don't, you need to rename the second sheet or copy the contents into a blank sheet, then relink the correct sheet to the new files
Repeat for part 3
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