Sure, it could be possible in the comics. The comic - book realm is full of what - if scenarios. Given the right writer and storyline, a love story between Captain America and Black Widow could have been a great addition. It would have changed the way we view their relationship. They both have strong personalities, and seeing them in a romantic relationship could have led to some interesting character developments and story twists.
It might have been possible. Comics are known for their flexibility in story - telling. While they are mostly shown as friends and allies in the regular story arcs, the vast and diverse nature of the comic - book universe means that in some off - beaten - path or alternate reality stories, a love story between them could have been crafted. It would have added a different dynamic to their characters and the overall story.
Yes, it could have been possible. In the comics, there are often alternate universes and different storylines. So, in some of these alternate takes, a romantic relationship between Captain America and Black Widow could have been explored. However, in the mainline comics, they are usually depicted as comrades.