
I haven't read novel (I plan to but haven't yet). For now, as we know it the mc and Junwoo, his old crush in the ambulance, were only just ever friends. Mc would also try to be useful for Junwoo to gain favor even though he didn't really expect much from it. (Example is lying to Junwoo's father for him.) Junwoo was never aware the mc had feelings for him and still isn't aware, the mc never plans on telling him either.
As for Taesan, we know Junwoo has a very bad and possessive personality, especially twords Taesan because he's attracted to him. We know Taesan has positive feelings twords the mc (and personally, this is my own theory though, that Taesan actually has romantic feelings for the mc and Junwoo knows about this but the mc doesn't. I also somewhat believe this is related to what Taesan was going to tell him in the school shed before Junwoo interrupted, that's why Junwoo gets so angry whenever the mc helps Taesan because then Junwoo gets made out to be the villain, in which he is.. but yk, meanwhile the mc only gains more favor from him by doing so.)
Before Taesan catched Junwoo's eye, Taesan and the mc we're only ever somewhat friendly since he was a new kid at school I believe (unless I'm misremembering.) Their relationship wasn't that deep but it was positively friendly until Junwoo met him and instantly caught feelings leading to the intense bullying followed by the negative feelings from the mc (1 week no attention: ☠☠☠)

This is just purely assumption on my part but this is wht I see how their relationships go:
At the start, Jun(MC) likes Junwoo(trash) and is showing it by being helpful to him.
Then here comes Taesan. Junwoo likes him, but shows it by bullying him (I don't know if it's because he blames him for his turn out or that he's just a psychopath).
Of course, Jun catches up on to this. And thus tries to sabotage(?) Taesan and Junwoo's relationship by acting as Taesan's savior.
However, it backfires because by doing that, Taesan ends up liking Jun. Junwoo didn't like that, thus beating Jun.
After the squabble, Jun realized that liking Jun is not worth it (at all).

Of and forgot to mention this, but I think one of the reasons Junwoo is bullying Taesan is because he wants to isolate him from his classmates. If Taesan is isolated from the class, it will be easy for Junwoo to carry out his whims on the poor guy. But of course, Jun gets envious of the attention Taesan gets out of Junwoo. So he counters it by being the only one to help Taesan from being bullied by Junwoo. For all I know, Jun only do the saving by the chance that he gets more of the attention Taesan gets out of Junwoo. Well, he gets more than what he's bargained for and finally snaps back to the realization that liking Junwoo is not worth it, at all.
Can someone plz spoil me what the relationship between Taesan and MC’s (past?) crush who’s name I don’t remember and just got sent in that ambulance. Like why is he picking Taesan over MC and why is he upset when Taesan even interacts with MC to the point that he beats him up. Why would he start constantly abusing Taesan instead of just playing along with MC’s feelings in the beginning? I have the feeling that their relationship somehow started with him developing romantic feelings for Taesan and tearing him poorly because he can’t accept it himself and Taesan does his best to try to keep the peace. I have this really bad feeling that he has been s*xualy abusive towards him too and I really hope I’m over analyzing this.