Futago Kareshi
https://www.mangago.me/read-manga/futago_kareshi/
Thank you for your assistance kind sir
Hanamaru Inkya na ga Futago Boku ni Aisareru Iyau
https://www.mangago.me/read-manga/hanamaru_manga_inkya_na_ga_futago_boku_ni_aisareru_iyau/
A Dead Crow's Stare Webtoon
https://www.mangago.me/read-manga/a_dead_crow_s_stare/
Mob-kei Danshi no Yuuutsu
https://www.mangago.me/read-manga/mob_kei_danshi_no_yuuutsu/
https://www.mangago.me/read-manga/tachi_to_neko_docchi_ga_ii_no/
These in these two, the uke isn't familiar with them from the start but just in case you'd like to check them out anyway:
https://www.mangago.me/read-manga/i_a_kept_man_am_the_pet_of_two_high_spec_men/
https://www.mangago.me/read-manga/can_angels_do_this/
a classic right here
https://www.mangago.me/read-manga/from_points_of_three/
Gee. It would be a shame if someone recommended me a good threesome BL story. Yk id also hate it if they started off as friends.. just sayin