Trisexual is the attraction to any three genders. Trisexuality falls under the polysexual or multisexual umbrella alongside quadsexuality, bisexuality and polysexuality.
History
Trisexuality has had several definitions, both of them have been completely inaccurate and the last of which, completely immoral. The first definition was known as "an alternate name to pansexual", which implies there are only 3 genders. Another of the two outdated definitions is "attraction to men, women, and animals" which is wrong as non-human animals cannot consent to intimate acts.
The flag was changed because the previous flag followed the definition of "an alternate name to pansexual" which implies that there's only three genders.