Here's an excellent thread by an Indian trans women tearing apart the seminal anthropological text that has cemented the idea of Hijra "third gender"ness for its racism, orientalism, and transmisogyny.
https://twitter.com/talia_bhatt/status/1779895088266592638?t=HKXcxNoIXgo0pgWcMR-mzQ&s=19
Hijras are primarily considered "third gender" (which is similar to many other culturally embedded trans women in the global south being seen as a third gender) because western anthropologists and later Indian anthropologists uncritically accepted the degendering and marginalization trans women experience as ontological evidence of a third gender which was later taken up by state policies.
This has been done by ignoring hijras for decades who have self identified as women and using transmisogynistic talking points such as a lack of their wombs making it impossible for them to be women, their forced prostitution and begging because of their exclusion from the formal economy as "cultural practices", and their experiences detailing how despite their wishes and efforts society refuses to see them as women as evidence that they are not women. The vast majority of hijras are women; they exist as women, they take HRT, they get their legal names changed, etc. Calling them a third gender is structural transmisogyny.