The true ending is the one where Ichika runs from wedding because she fears that he does it out of obligatory feelings, while loving Youko. But they end up understanding how she feels and just let her cool off, then ask to join them, which she is glad to hear...
From there authour explains he gives 3 bonus endings, favorable for each of them, thats why they are titled Cases1,2,3. If you read between the lines it would be like authour apologizes to readers and says he doesn't want anyone to be hurt not readers not 3 protoganists.
Add that this whole story is motivated by realism concept in a certain way. From daily lives to psychological perception of each character and societal reaction on their relationship.
Whatever the ending you choose or come up with, just giving it a good thinking you will realise that one things is certain - all three of them had a happy end. Plus I should add that I personally like that ending, it makes you think. And if some things are hard to understand that means you are yet to expirience some human relationship or meet certain type of people.
P.S. You are welcome (▰˘◡˘▰)
The ending is absurd? What does that mean?Is he married yuoko or not?