So if you don't say what play it is, you could at least say how many lines or pages the biggest part has....guess how long it would take you to learn the part, then triple the time for a middle schooler.  Remember, that your camp could have rules that include "homework" for those cast as principles, so they learn the lines on their time.

 

Here's the catch with kids--short attention spans.  While you work with one set of kids, you may want a partner to be doing something with the rest.  Most directors of kids that I know separate the kids into predefined groups each run by an adult or competent Teenager assistant.  So while group "A" is working with the musical director, group "B" is learning the dances, and group "C" is learning blocking.

 

(You also didn't mention whether the 3 hours a day were for one day a week or 5 days a week.  If you meet with time inbetween allow a bit of room for kids to forget)

 

Finally, a suggestion.  Maybe some fun ideas can help you.  Consider as part of the activities a "race" to learn your lines.  You can separate the principles from the rest in categories and offer a prize for the winners in each category.  Similar contests can be held for least errors in blocking, most expressive face (offer this contest early, because 3rd graders can mug for-ev-er), best homemade costume, funniest player, funnest person to work with, most improved.  Your prizes can be as simple as a paper plate award.  You have a set of kids with plates and markers (Call them the 'critics') in charge of coming up with the awards for the day and presenting them--do tell them they have to be positive, and appropriate..