This set contains three of the five Sovereign denominations (minus the five-sovereign piece and the double sovereign) issued in 2021. All coins feature the special crown privy mark to celebrate the 95th birthday of Queen Elizabeth II.
The Royal Mint says about the set:
"The Sovereign has been struck for every British monarch of the last two centuries, including Her Majesty The Queen, marking many important moments in her long reign. For this 95th birthday year, The Sovereign 2021 features an exclusive ‘95’ privy mark and its milled edge also has a symbolic 95 mills. When one of the most prestigious coins in the world features a distinguishing mark, it becomes even more desirable, and this Sovereign set contains three of these special editions."