Transgender-threesome is a combination of two distinct adult content themes: 'transgender' and 'threesome'.
In simple terms, it means a porn video involving three people where at least one of them is a transgender individual. A transgender person is someone whose gender identity does not match the sex they were assigned at birth. This could refer to a transgender woman (someone assigned male at birth but identifies as a woman) or a transgender man (someone assigned female at birth but identifies as a man).
A 'threesome' is a sexual encounter involving three people. This typically implies a scenario where two people engage in sexual activities with a third person, but can also refer to a situation where all three participants are engaged in mutual sexual activity.