Add the following three (3) subgenera to genus Trillium:

Trillium subgenus Trillium
Trillium subgenus Callipetalon
Trillium subgenus Delostylis

Move the following species-level taxa into the appropriate subgenus:

Trillium subgenus Trillium (16): T. apetalon; T. camschatcense; T. cernuum; T. channellii; T. erectum; T. flexipes; T. × hagae; T. hibbersonii; T. × miyabeanum; T. rugelii; T. simile; T. smallii; T. sulcatum; T. tschonoskii; T. vaseyi; T. × yezoense.
Trillium subgenus Callipetalon (5): T. crassifolium; T. grandiflorum; T. nivale; T. ovatum; T. scouleri.
Trillium subgenus Delostylis (4): T. catesbaei; T. georgianum; T. persistens; T. pusillum.

At the end of this process, only five (5) taxa will remain as direct children of genus Trillium:

Ungrouped (5): T. × crockerianum; T. govanianum; T. × komarovii; T. taiwanense; T. undulatum.

This is the first step of a larger effort to revise genus Trillium on iNat.

Thanks @ddennism!