First, I would plan a meeting asap. I would send a note home with the kids like others have said describing what the Fall Festival is, what is done, what the profit is used for, etc. I would then give a brief description of areas that you need help. Make it positive, add that if everyone will dedicate 30 minutes that night to help then everyone will be able to enjoy the festival with their families as well.
On the same note, I would have a date and time within the next week for a Fall Festival meeting. Offer refreshments or whatever if you have to. Mention that turnout at the meeting is crucial if the parents are interested in having the festival because it will be cancelled if there is a "lack of interest". On the note, list other ways to contact you or others over the festival if they can't come to the meeting. Make sure they have your phone number, email, fax, tell them they can send a note through their child, etc.
At the meeting, divide them into classrooms. Ask for one or more people to take responsibility for contacting the parents in their class and scheduling shifts for a particular booth, game or area. Make sure you have the names and phone numbers available to hand out at the meeting as well as a form that they can use to write down who will do each shift. Our school gave us the classroom information we needed.
Have a list of jobs that need to be done. Break it down into manageable parts. Some people can pick up donations after work, some can make phone calls, some can help with set up. This way they can help without feeling like they'll be responsible for a huge amount of work.
Once you have quite a few classrooms spoken for at the meeting, send a note to the teachers that don't have any volunteers from their class. Ask them if they have room parents that they think would be interested in helping you. Most teachers will want to make sure that they have participation if everyone else is doing it. Don't make the teacher responsible for it, just see if they can recommend someone. Call that person and tell them that the teacher recommended them because they were always so helpful and willing to do things when needed, etc.
Let other people make phone calls for volunteers besides you. When you get others to help, they have their circles of friends that they can twist their arms, grandparents, other parents that might be willing to help if asked by the right person.
You have a lot to do in 3 weeks, but if you can get the volunteers soon I know you can pull it together. Just remind everyone that the kids love the festival and look forward to it and you want to be able to continue the fun this year.
Carrie