Lately, I've been into those threesome mangas, but I'm getting a little tired of the repetitive concept of two semes doing the uke. I don't mind two semes or two ukes or whatever, but what I really want, is to see a relationship where all three of them are actually involved.

Not like:
The semes love the uke and get into a relationship with him, having no feelings for each other whatsoever.

More like:
No matter what 'role' or 'position' they have, everyone is more or less equal. The closest I've come to find something like that is Friends Like A Lover (more implied than shown) and Goshunjinsama to Wanko (where seme 1 kisses seme 2 at the end).

I want a manga where I feel that they're all involved. Several chapters or a oneshot, it doesn't matter.

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in need of romance?
uncommon, sad, sweet

Have you read Futari no Musuko ni Nerawarete Imasu yet?

Futari no Maharaja it does look like it at first but they don't really hate each other its a misunderstanding.
Hard Core Heart
Koi Tomo

Everything's Yaoi in My Mind!

I read some of the suggestions, and I may not have been clear. I want something where ALL of them love each other, like: 1 loves 2 and 3, 2 loves 1 and 3 and 3 loves 1 and 2. Simple.

To be honest, I don't think you're going to find very much of this in manga. The BL genre is pretty firmly entrenched in the seme/uke dynamic, and menage/poly manga is also pretty rare (not counting hentai). Finding something that meets both of your requirements is a pretty tall order. You might consider branching out into reading western m/m prose novels instead. Goodreads has a pretty big list of user voted books with this theme. One that I personally like and recommend is The Hot Floor, by Josephine Myles.

Good luck, and I hope you find something enjoyable.

I see. Well, I might crave a little too much too soon, haha. I'll check it out, thank you for commenting.

Bachi Bachi (KIJIMA Hyougo), the first and fourth stories, will make you happy
Fallintrash, a oneshot
Ikenai Otoko, maybe. By the end there is pretty much one established couple, but given the nature of what they do, I'd say a repeat event is likely.
Groupie

until it falls apart at the end BTW, this is one of the very few works by NITTA Youka that I did not like. 😢

This is really late, but I guess Futari no Musuko ni nerwarete imasu 😕 ?
It's about two brothers who like their dad (not related by blood)
They both love the dad and both of the brothers end up with him, but I think the brothers like each other in a non incest way