Bakugo is not a man of many words, and the words he does have to share are often for the sake of protecting his ego, so paying attention to what he does will really let you understand him better than anything else. For example, when he suddenly takes up the habit of putting Deku in a headlock while he yells at him instead of trying to beat him up, nobody is quite sure what to do, because it is technically less violent, but it’s hard not to still worry.
Bakugo wants to touch Deku, but that doesn’t make any fucking sense, and beating the nerd up all the time keeps getting them both in trouble, so headlocks seemed like a sensible compromise. Sensible compromise, Uraraka. Not all of us want to go around with our feelings on our sleeves or some shit like that. Not that he has any special feelings towards nerds like Deku.
And if Bakugo slowly but surely starts getting into Uraraka’s space more often, with more and more close or physical contact, and for longer periods of time, it’s not until the habit is already formed that anybody realizes what’s going on. Uraraka probably knew, but she’s not going to scare the idiot off when his subconscious is finally getting the better of him. By the time she starts to get into his space, he’s in her space so much that almost nobody really notices the difference.
Deku notices because he and Uraraka have been attached at the hip almost since they met. Before the dorms were a thing, they walked home together even when Iida had other plans after school. Quickly they began to feel as if something was wrong if one or the other didn’t spend their lunch period together. Deku notices because he and Uraraka have been in each other’s personal space from the beginning and Kacchan getting into her space means that he’s basically in Deku’s space too.
Deku suddenly starts to notice he has a bit more personal space when Uraraka starts getting into Kacchan’s space, and this is fine. Really.
If Deku starts to not just be next to her, but gets into her personal bubble, and Kacchan suddenly has cause to pick fights with him again, well, it was bound to happen.
Bakugo puts Deku in a headlock and drags him around by the head, but when he realizes that this makes it harder to impose on Uraraka’s personal bubble he comes up with the brilliant idea to just snake his arm around her waist and put her in his own space. He knows this is a different thing with different implications, but it was always coming down to this anyway. He’s saving everyone involved the time and emotional confusion. He’s doing them a favor! Be grateful, nerds!
One day he has his arm around Uraraka before he comes across Deku, and well, the headlock would require him to let go of her and that’s not happening, so he grabs hold of Deku’s arm and drags him in close. Bakugo’s face doesn’t show any sign of doing anything weird, but he looks grumpy, so neither Deku nor Uraraka say a word about it. And if Bakugo just keeps grabbing Deku and Uraraka and holding them close, well, he’s always warm and surprisingly more comfy than you’d expect, so neither of them is complaining.
Deku and Uraraka somewhere along the line got in the habit of just talking around him unless he demands their attention. They both go about life as if Bakugo isn’t snuggling them, talking to other friends like normal, doing homework, watching movies in the common room and falling asleep on him.
Nobody is surprised when Deku and Uraraka begin to be found holding hands and on each other's shoulders and laps. But if they expected a jealous Bakugo, they are disappointed when he just finds a spot between them or on either side of them. They always make space or have already left a spot free for him.
So it makes sense that at some point he stops having to reach out to grab them because they grab him. Bakugo then finds he can hug Uraraka from behind with Deku riding on his back and the other way around. On one such occasion, Deku rests his chin on Bakugo’s shoulder and drops a kiss on top of Uraraka’s head when Bakugo gets his arms wrapped around her and tucks her into his chest. When Bakugo immediately looks like he’s about to make a stink about it, Deku gives him a peck on the cheek and, unsurprisingly by now, Kacchan just lets himself begin to grin before hiding his face in Uraraka’s hair.
Nobody knows if the three of them have talked about whatever is going on between them, but all three are the same people they were before it started. This means Bakugo is still a volatile dick, so nobody has worked up the nerve to push the issue yet. It seems like the kind of thing you’d have to have talked about, but weirder things have happened.
Deku and Bakugo have always understood each other when it matters, and Deku and Uraraka never have seemed to have trouble communicating what needs to be communicated, while Uraraka understands the many expression and deflections of Bakugo well enough to make their one-sided conversations two-sided enough for the relevant parties involved.
When Deku and Uraraka start to take turns invading Bakugo’s lap when he’s trying to do other things everyone gives up entirely on worrying about it. Obviously whatever they’re doing works for them, so it’s best to let well enough alone.
The three of them pass out in mini-puppy-piles around the dorms often enough that everyone has seen the triplicate matching content smiles before covering them with a blanket. It’s disgustingly cute, and it makes zero sense that someone so rude has this big of a soft spot, but love has a funny way of blowing expectations away like that.