The reason why three reports are required is that all three credit reporting agencies do not have collated data amongst each other.
This means Transunion would not have merged data with Equifax and Experian and vice versa.
These residential mortgage credit reports are generated by companies that are specialized in merging the data from all three credit reporting companies and provide collated data for the applicant’s and co-applicants FICO scores and credit reports hence they are called tri-merge credit reports into one master credit report.
These companies are called mortgage reporting companies or intermediary bureaus. This master report is used by mortgage lenders to determine credit qualifications for applicants.
This report is generated in a very unique format with multiple pages which is compatible to be read by both humans and systems.
These reports are bifurcated into sections where you have all the information about the applicants including scores from all three bureaus, the latest credit checks done, and details about all your credits with the date when you opened and your payment history on each one of your credit products.
The format of the report is made in such a way that it could be inserted in the systems for underwritings as well as could be read by humans as well.