The idea is for three Democratic presidents — Biden, Bill Clinton and Barack Obama — to appear together at a fundraiser this spring, the four people familiar with the discussions said. It’s also just one in a growing list of ways that Democratic leaders, and the Biden campaign, are gearing up for a general election they view as having the highest of stakes. The Biden campaign did not respond to requests for comment. The Biden campaign in recent days shifted into a general election posture — earlier than the president’s aides anticipated. But the Biden campaign also hopes the presidential trio will help mobilize the party base.