Hello,
If you set mergeType as 1, only the first section is kept; otherwise all sections are kept. All DOCX must have at least one section.
If you send to contact[at]phpdocx.com the three DOCX you are merging the dev team will merge them and check the merged output. DOCXPath is very useful to modify document contents of a DOCX, so it may be helpful to accomplish what you need to do.
Regards.