
like almost every typical mainstream shojou manga ever

Honto Yajuu has one...I think its a bit later, towards the middle, and maybe its more like a rival shows up. But its adorable...worth you reading regardless.
http://www.mangago.zone/read-manga/mujihi_na_otoko/ plus you know, its super good, has two sequels, and there's related stories. I'm sure you've read but just in case.
http://www.mangago.zone/read-manga/dog_style/
http://www.mangago.zone/read-manga/19_days/ plus you know...cuteness induced hives.
these are yaoi now for some *good* shojo :P
http://www.mangago.zone/read-manga/beauty_pop/ plus the heroine is super cool...
http://www.mangago.zone/read-manga/hatenkou_yuugi/ its action adventure fantasy romance...super cool I feel.
http://www.mangago.zone/read-manga/tenshi_ja_nai/ more traditional shojo...with a male genderbender
http://www.mangago.zone/read-manga/shiawase_kissa_sanchoume/ well you say triangle...
http://www.mangago.zone/read-manga/koi_dano_ai_dano/ plus the heroine is super cool and gonna rule the world...without anyone realizing she's doing it.
well try those and if you need more I can assist

Hydra serie

right...I mean for the most part it hasn't happened to me with manga because they develop the character they eventually pick better but its happened. Boys over flowers I actually really liked the first dude...but I didn't read the manga. And actually itzura na kiss ..."I'd have picked ducky" *his names not ducky but its the ducky character...don't remember his name. But she should have picked him. So for the most part I read mangas without love triangles and I stop reading if I don't like the main dude because he's an a-hole. Probably do that with love triangles too...if she's clearly going to pick the wrong guy, I'm like "aaaand she's stupid and I am GONE." unless its super good and worth it. Reason I didn't finish Moe Kare...
Now I'm pissed about Ducky again
Does anyone know a good manga with a love triangle?(▰˘◡˘▰)