XD well with your specificacions let me tell you about 3000 mangas like that XD
Elektel Delusion
Here are some random titles:
http://www.mangago.zone/read-manga/rift_conro/
http://www.mangago.zone/read-manga/liquor_cigarette/
http://www.mangago.zone/read-manga/hitorijime_no_jouken/
http://www.mangago.zone/read-manga/mahoroba_days/
http://www.mangago.zone/read-manga/koisuru_taka_wa_tsume_o_kakusu/
http://www.mangago.zone/read-manga/toshigoro_no_otokonoko_to_are/
http://www.mangago.zone/read-manga/orutana/
http://www.mangago.zone/read-manga/koi_o_hitokuchi/
http://www.mangago.zone/read-manga/hima_na_no_de_hajimete_mimasu/
http://www.mangago.zone/read-manga/itsuka_tomodachi_ja_nakunaru_to_shite_mo/
http://www.mangago.zone/read-manga/jibun_katte/
http://www.mangago.zone/read-manga/yakozen/
http://www.mangago.zone/read-manga/soredemo_yasashii_koi_o_suru/
http://www.mangago.zone/read-manga/bokurani_matsuwaru_etc/
http://www.mangago.zone/read-manga/by_my_side/
http://www.mangago.zone/read-manga/waraibanashi_no_you_da/
http://www.mangago.zone/read-manga/tsuioku/
http://www.mangago.zone/read-manga/itoshi_no_nekokke/
http://www.mangago.zone/read-manga/boku_ni_datte_iibun_ga_aru/
http://www.mangago.zone/read-manga/saburina/
But anyway, ALL from the #close friends or #childhood friends tags fit your description. As anon said, 3000 mangas isn't exaggerated.
I search for yaoi mangas where best friends become a couple but at the beginning that is just a one side love