Of course he'd known about Clint. What he didn't know was that things had progressed further than the sort of conversation they had on the net where they kinda put some of it out there and left things open ended on everyone's part. That was the best way to do it, he'd thought at the time and still held that belief now. It allowed them all to feel what they needed to feel and figure out what to do about it. From the sounds of it, she and Clint had begun to figure it out.
The feelings that begun to resonate in his broken heart actually surprised him a bit. Bucky felt a bit of longing, absolutely. That longing could've spiraled downward hard and fast, into him becoming distraught over missing out on telling not just one but two people how he felt. The thought did occur to him, but he didn't resent them for that. If anything, it was just on him. He'd stewed around on this whole situation with Steve for so long that it'd prevented him from exploring something to the fullest that could've been amazing. But that was on him, and he could accept that no matter how much it hurt.
Beyond that? He felt a sense of happiness. Natasha and Clint were both amazing people. People he cared about deeply and held in such high regard, who he had feelings for himself. His feelings, he knew, were genuine. With that being the case, how could he be anything but happy for them? He knew Natasha better, knew where she came from and what she'd been through. Of course she deserved to be happy. And Clint was just fantastic. Bucky wanted it for him, too. He didn't want to go rage punch him for stealing Natasha. It wasn't even like that. She wasn't an object for them to compete over or fight each other to win. They were both people he adored.
Bucky lifted his human hand and passed his fingers slowly through her hair, from red to blonde, until he reached her shoulder and let his hand gently rest there. "Good," he said. "I want that, for both of you. You deserve that, so much."