Tenshi Dattara Yokatta

Kako, a first-year high school girl, has been friends with Taisei and Yuna since elementary school, and she wished they could always be "the three best friends" forever. But the fragile and precariously balanced relationship crumbled when Taisei got injured, and Taisei and Yuna became a couple during his recovery. And then, a mysterious boy who met Taisei while he was in the hospital appeared...
(Source: Betsuma, translated)
Note: Includes 1 extra chapter.