United are on course to win the English Premier League, FA Cup and European Champions League and emulate their feat of 1999 but a spate of injuries to key players is causing problems.
"We must have a chance but it's a very difficult one, more difficult than it's ever been," Ferguson told a news conference to announce United's tour of North America this year.
"It's more difficult than it was in 1999 because in 1999 we didn't have an injury through the last three months of the season," he added.
"That kind of luck is, at the moment, deserting us because we have so many injuries, particularly to defenders."
Ferguson admitted he would settle for just one of the trophies.
Arsenal are snapping at their heels in the Premier League, United face Manchester City in the FA Cup semi-finals and Chelsea in the Champions League quarter-finals.
"Going away from the greed of achieving three trophies, if you win one major a year then you have to be happy with that," Ferguson said. "If we can win one of those big trophies I'd be delighted."
But Ferguson said his players were still committed to the ultimate challenge and that they enjoyed a major psychological advantage over their opponents.
"There's a great desire to do it. Given the opportunity to achieve that type of target, we'll always try to take it," he said.
"No matter what the scoreline is, you always feel Manchester United are going to score in the last minute. And that's a fantastic advantage to have."
Ferguson said he hoped to improve his squad during the next transfer window.
"We have certain targets in mind at the moment," he said at the home of New York's Red Bulls. "I think two or three will be fulfilled, there's no question about that."
