The three young Royal family members each set up a firm in case they ever need them "to protect intellectual copyright", a royal aide was quoted as saying by The Telegraph.
The firms were set up in 2012 on the advice of lawyers, but have been dormant ever since and are not intended to be used for trading, the report said.
In a move more often associated with celebrities such as David and Victoria Beckham, they told lawyers to establish companies in each of their names.
31-year-old William's company is called APL Anglesey, which is made up of his middle initials - Arthur Philip Louis - and the island where the couple lived at the time.
Its main director is Miguel Head, the Duke's private secretary.
Kate, 32, used the name CE Strathearn, her own initials, Catherine Elizabeth, and Strathearn from the Countess of Strathearn, one of her official titles, and chose her own private secretary Rebecca Deacon as a director.
Prince Harry named his own company after an African gazelle and nominated Jamie Lowther-Pinkerton, who is principal private secretary to all three, as a director.
A spokesman for the three likened the companies to the Prince Charles' AG Carrick company, which is used for the sale of prints and photographs of his watercolours to raise money for one of his charities.
"It is a pre-emptive move to give them a vehicle for intellectual property, if they needed to trademark anything in the future, but there are currently no plans to use them. They are not intended to be used as trading arms," a royal aide was quoted as saying.