Yes, I think there's a huge step between realizing you're not able to be an aggressively evil DE or kill people, and taking a step to undo what you've done. Regulus, of the three, is the one who seems to have taken his own redemption in hand and sacrificed himself to right a wrong--though as Jodel points out above, we don't yet know that his change of heart really was that. For all we know he found out Voldemort was a Half-Blood and tried to destroy him in a fit of Pureblood Supremacy Idealism!

Snape we don't know yet exactly what his line of thought was, but obviously it didn't cure him of his old hatreds or being a bully. I think people are wrong to judge Draco's frozen state at the end of HBP as just cowardice or lacking guts because I think it seems so clearly more than that, but it's not redemption either. It's funny, in a way fandom sometimes just has a trouble accepting that a change requires change--it doesn't just happen all at once. You don't just change on a dime from one person into another person. Each of these three, if they change(d) their minds, would have to go through a process.