Is it possible to have a character that is still majorly significant in a book but not a main character? Like, they don't appear every chapter and they don't really have too much to do with the MC?
I think 'main' classification is kind of arbitrary and up to you, really. There are terms like protagonist, deuteragonist, tritagonist, but you can assign meaning and importance however you want (there are dual protags, after all). So, sure. Although if they don't have much to do with the mc, I would wonder how much they have to do with the plot, and whether they're really important -- but it depends on your story, of course. It's certainly not impossible.
Most stories have more than one important character no matter how you want to classify them or what you want to call them. Are you really asking if you can have important characters other than your MC? Or am I missing something because I don't really see how this is something that would confuse a writer?
I have a situation where there’s a character who’s supposed to be one of the three main characters, but everyone who’s read for me says he’s not a main character. I’m just trying to figure out if what I have for him marks him as significant rather than main, because I’d probably be okay with that. He really is important to the story, so that’s why I’m concerned about his status as a character.
You're probably getting that feedback because he's not in the story much, it sounds like. If he's really a key player in the plot, you might try beefing up his role some. What role does he play, if it doesn't have much to do with the mc? That's where your reader's main interest is going to lie, so if he's not important to the mc, I can see why he wouldn't feel important to the reader.
He's one of the antagonists, though not really the only one in the whole book. Also, he doesn't like the MC, even though they're twins. The MC doesn't like him either. So they're rarely around each other for any interaction to happen, which I think is the root of my problem. That's what I'm trying to do right now, because I don't particularly like relying on what I have until I know I'm 100% out of ideas. I'm thinking maybe I need to lower the hate between the two and allow for them to have some desire to be normal twins who care about each other, even though they're so different.
That might end up making him seem even less important, though. If there's not as much tension, what is important about him? Are you going to derive more Plot/Content from them trying to be friends?
I'm thinking maybe a subplot of the MC secretly wishing he had a better relationship with his twin and vice versa would make him seem more important. Then let them have a taste of it for a while before going in and having one of them ruin what they built up, which wouldn't be a whole lot due to time. I have basically no way to get them together as it stands right now. They can easily get out of pretty much every situation where they would have to tolerate each other. It's all just hate, hate, hate, and I can't really get a subplot out of that.
That sounds cool. The pacing might be tricky since you say there's not a lot of time. I'd try to make sure you're doing everything you plausibly can to give him as much positive screentime as you can so it matters more when it goes bad.
What about them having a friend in common, who’s torn between them and also sometimes negotiates between them? And giving the less important twin something that the important one really needs, so that the negotiation is needed?
Personally I don't much care for changing the natural flow of the story to better fit a labeling convention. If it makes the story better to have more than one perspective at play so be it. If it makes it better to give a particular character more or less screen time so be it. I don't think it matters so much who is a main character and who isn't. A better question is what do you want to focus on to tell a better story?