Disclaimer: This headcanon ties specifically to my version of Steve, and the dynamics shared between these three as a result of his backstory and theirs.
While they can come tagged as 'set: do not separate' in truth much of this can transpose onto other Steve players provided they are 1) okay with that and 2) either genuinely living a normal life or, like my Steve, projecting the image of being Fine(tm) like his life depends on it.
Carol and Tommy have known Steve their entire lives. Consequently, they knew his family - Tommy was just like Steve when it came to idolizing and also half-hating Mark for blazing a trail that felt impossible to follow.
His grades and his skill at sports were dauntingly impressive, but he always had time to look over their work or play games with them, helping them get that edge early.
Carol, meanwhile, adored Sandra and genuinely felt like she had an older sister in her.
Both of them knew and even helped babysit the twins. In a lot of ways the Harrington house was their second home - they were there more often than they were their own homes, for various reasons - and when tragedies began to strike, fractures formed at the very foundations of their friendships.
It all started when they were twelve. Something bad happened to Tommy, and Carol was the sole witness. Police got involved, and Tommy was in the hospital for a few days, but Steve never heard about it - nor did he hear about the fallout between Tommy and his dad that lead to Tommy becoming effectively homeless.
Six months after that assault, Tommy was invited to live with the Perkins family - specifically by the matriarchs, Rita and Rose. Both Tommy and Carol agreed not to tell Steve about this, because Tommy didn't want Steve to know what had happened, and Carol - selfishly, at the time - didn't want Steve to insist Tommy stay at his place - she liked feeling needed, and she knew Tommy would prefer the Harrington house to the Perkins one.
Tommy had been living with the Perkins and holding in his secrets for almost a year, when Rita Perkins was killed. Like Carol, he didn't believe for one minute that it was an accident. The anger and pressure inside of him began bubbling over, and it was at this point he began building a reputation for lashing out violently.
Carol, meanwhile, was a torrent of rage and grief that was hard for anyone to handle, especially after her grandmother left mere months after her mother died. Left alone in the house with her father, who was turning to drink in his depression, and Tommy, who was as angry and full of hurt as she was - they both began to descend into a dangerous sort of codependency.
When Carol came home covered in blood and shaking from what she had done, Tommy was the one who led her by the hand into the shower and gently washed everything away. He was the one who pulled her back to herself, and learned her secret. They were bound by the fact they each had the knowledge to destroy the other, and they knew it.
Their coping mechanisms ranged, and were not always healthy. Shutting Steve out wasn't conscious or intentional - but it was something Steve noticed even before his tragedy came into play. He noticed that his friends were angry, and volatile. He wasn't blind to the fact that he seemed to be the one person they would listen to, and it scared him.
He didn't know how to help, or even address the way things were - and when the twins died, he didn't dare bring it up. He was terrified of losing more people in his life - his house used to be so full it was overwhelming, and now he lived in it alone and barely knew how to function.
He couldn't contend with the idea of losing Tommy and Carol too, so he enabled a lot of their behaviors and at times encouraged it, took part in it, hell even instigated it - because there were times when he got it.
He got how comforting it was to lash out and make someone else feel like shit - because for a little while, you weren't thinking about how shit you felt inside.
Meanwhile, Tommy and Carol still thought he was better than either of them. Kinder. As far as they could tell, his grief hadn't made him crazy or violent like they felt half the time. He was still gentle in ways they felt they couldn't be - and rather than want to destroy it, so he could be just like them - they were determined to protect it, to the point they unconsciously began pulling away even more and adding to Steve's stress until finally, things snapped.
Steve walked away because he felt it was necessary - like Carol and Tommy didn't want him around, and Tommy lashing out at him was proof; because never once, in all his rampaging, had Tommy ever turned on him.
Carol and Tommy let him go, because at first they thought he just needed time to cool down. And then, when the miscommunications reached points of no return, because they could fathom no other option. Even mad and hurt as they were, the concept of attacking or hurting Steve was beyond them.
He's still painfully important to them, even when they aren't active in his life. Years after he has walked away from them, they are both ready to lay their lives on the line for him, because he never stopped being Steve. His ability to be hurt and stay kind is something they don't understand, but respect and admire enough that they'll fight for it tooth and nail, every time they reconnect with him.
&& THIS IS TRUE OF ALL STEVE INTERPRETATIONS. They are always willing to fight for Steve; either because they know he has a pain he carries and manages to stay kind despite it, or because he hasn't known pain and they do not want him to.
