 If you liked
Choco Strawberry Vanilla
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 ...then you might like
Ai to Makoto
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If you disregard Ai to's Omegaverse setting, then you'll find that it and Choco have nearly identical settings - two brothers/childhood friends who share a dysfunctional relationship (that involves having sex with each other). Whenever a third party is involved between these two, things always turn sour, till one day the two finally find a special someone. This person (also our MC) becomes an unwilling part of the relationship the brothers share, including having physical intimacy with both, till the three learn to cohabitate with each other.
Bonus: Both manga have sexy threesome scenes.