It's horrible that such a lovely story get this horrible attitude in the comment section. So let me straighten this out:
None of the characters did anything wrong, especially not Ritsu. You cannot judge her actions without taking the difficulty of homosexuality into consideration. Ritsu was at a horrifying disadvantage compared to Shun for being a girl in love with a girl, thus she could not confess to Ichika before Shun. It took a tremendous amount of courage to finally decide to tell the truth, and a galatic amount of luck to have Ichika loving her back too. In the real world, the most likely scenario is that the lesbian will keep her mouth shut and watch her crush get snatched away, or confess and get rejected.
Shun did the right thing by encouraging Ritsu to confess, because he wanted to fight for love fair and square, instead of taking advantage of the whole heterosexual privilege (which enabled him to comfortably confessed to Ichika in the first place). He also realized Ichika does not like him that much, so trying too hard to keep her would be a dick thing to do. Do a thought experiment: what if Shun had been a girl and Ritsu been a boy? Do you think Shun would have been able to date Ichika first? No. Shun would then be the one who painfully watches Ritsu and Ichika going out without daring to do anything about it, just like Ritsu. Shun understands that, so he wanted to treat Ritsu fairly, because that's how he would like Ritsu to treat him had the situation been reversed.
Ichika also did nothing wrong. She loved Ritsu but had no idea about homosexuality, so she just innocently go out with Shun because he's a close friend AND a boy, while staying best friends with Ritsu. She didn't like kissing Shun. Only after Ritsu kissed her, the possibility of romantic love with a girl opened to Ichika. And it took her a little time to find the courage to accept homosexuality, break up with Shun, date Ritsu.
Nobody has to feel bad for Shun, because this is the luckiest outcome he has got. Ichika didn't love him, so their breakup is inevitable in the future whether Ichika learned about homosexuality or not (and Ichika will go on dating other boys trying to find the romantic feelings that she'd never be able to get), or Shun would have married Ichika without ever being loved by her. Both scenarios are much, much more miserable. You can't have what don't belong to you. Shun understood that, so he let Ichika go, preserve the friendship of the 3 instead of being a childish fucker who would whine and get nasty about for not getting what he wanted. It's a fair fight and he lost, get over it.