Well, in the comments he wrote that he activated the Field Magic card "One Husband Many Wives" that allows his marriage with all three Blue-Eyes to be accepted haha
(Pronunciation)Date of birthPhotograph or PortraitOccupations of applicantsHusbands OccupationHusbandWifeThe next president and CEO of Kaiba Corporation.SetoWhite DragonFull namemarriage registrationWhite DragonWifes OccupationBlue EyesBrideto beRegistration StatementHusband to beKaibaSetoLastFirst2008 November 28th.KaibaMy supreme S servant isn'tt bound by its date of birth!KaibaBlue EyesOctober 25, 19XXSignatures and Seals of aplicantsHow you met and fell in love with your wife (husband)OtherIn my sophomore year of high school, I had a fateful encounter with Blue Eyes in the humble game shop that Muto Yuugi's grandpa ran! And then that old man... (You know the rest)3000 years ago? ... How unscientific!A servant who's purpose is to inherit my pride and soulActivate: Field magic "Polygyny". My marriage to 3 Blue Eyes will be approved.Kaiba Seto