Can eating spicy food a couple of days in a row induce a crohn's flare up that lasts 3 weeks?

Depends: Depends on the severity of the Crohn's disease you have. In any event I would not tempt fate. Follow your physician's advice as to proper diet for your condition.

Yes: Crohn's disease is a chronic inflammatory condition and anything thing that triggers the inflammatory cycle can cause flare that lasts after exposure to the trigger is over.