Natasha did have a crush on Steve Rogers back in 1990s Avengers comics. And reader, I really hated it:
Natasha: Why do you insist on keeping your fellow Avengers in the dark? As chairman, I should be angry with you…but oddly, when I see you lying like that, I find it quite impossible. look at you— exactly the same as you were when I first laid eyes on you long ago— another world, really— in Madripoor. You are a very handsome young man, Steve Rogers… And for the first time, perhaps, I’m allowing myself to see that…
So, look. Steve Rogers is objectively hot. We’ve already seen that Natasha has had a long-time thing for Clint, and given that he and Steve look exactly the same, it makes sense that she’d also find Steve attractive. My problem isn’t even that Natasha’s one-sided crush is introduced via her recollections of first meeting Steve as a six year old, which is creepy and objectively not hot.
But this plotline was part of Bob Harras’s Avengers run, which should have been a great period for Natasha on the Avengers, but wasn’t. Like this text references, Natasha was the leader of the Avengers at the time, so it should have been great to see her making tough decisions and developing as a leader. Instead, she acts as Steve’s sidekick almost the entire time, being weirdly deferential despite technically outranking him. Like, she can’t get mad at him as a leader because he’s too pretty here, which isn’t usually something Natasha suffers from even when surrounded by very pretty men.
Probably the best example of this weird lowkey OOCness for Natasha through this entire era is when the Avengers fight over the use of lethal force, and Natasha unblinkingly takes Steve’s “no-killing” sides, when Natasha has been okay with using lethal force in basically every other appearance. She fought with Daredevil about this during their partnership, so this was basically worse than 1970s Daredevil in terms of giving Natasha a credible PoV. That’s not good kids.
Imo, the attraction of a Steve/Natasha relationship should be them both overcoming drastically different points of view and learning to work together and appreciate the other’s perspective. They should challenge each other. But when the idea was developed in canon, it was “Natasha goes along with everything Steve says because Steve is Always Right.” Thankfully, this was all dropped completely when a new writer came on the Captain America book and no one mentioned it again. That writer was actually Mark Waid! CBR did a retrospective of the dropped plotline a while ago if you are curious.
As for Bucky’s crush on Steve, that’s always been much more IC and well-attested to. Here’s some panels from Samnee:
Bucky: For the rest of my training, I couldn’t stop thinking about this ‘Captain America’— whoever her was. And I kept goin’ back every night to see more newsreels… studying his every move.
Panels from Avengers #382 and Captain America & Bucky #620.