All three companies use different scoring models. If you get a 3-in-1 report from transunion, they are using their score model on experian and equifax as well. That doesn’t give you an accurate score. You might pay more for 3 separate scores from 3 separate companies, but you would get the actual scores the creditors.
Also, not all creditors report to all 3 credit reporting agencies. So if an adverse account reports to experian and equifax, but not transunion, transunion would report a higher score. Most creditors will pull credit files from all 3 companies, and will use an average of the 3 scores, or the median score.

Each credit scoring company has a different way to measure your credit. They are all your "real credit scores." The same way that 12", 1′ and 1/3yd. are all the same distance, it’s just depending on who uses it. Most companies I’ve encountered go by my Equifax or Experian credit score. If you don’t know much about credit or credit scores, go to your local bank and talk to a financial adviser. They’ll let you know what a good credit score for you and what you can do to make it better.

Different credit agencies will review different aspects and have different criteria. It’s good to know all of them because different organisations will use different agencies.