We started learning about summary with 'Into the Book'. If you've never used this site it is a great tool to teach different reading strategies! It has great interactive lessons for the kids as long as a variety of teacher resources. It has quite a bit of stuff for summarizing and my kids absolutely love it! If you don't have computers in your room for kids to explore this site, you can always do it as a whole group lesson too. :)
My class seemed to grasp summary of fiction much easier than literary nonfiction. Literary nonfiction is a new genre to them in third grade and sometimes they have a harder time with the story. We read Cactus Hotel by Brenda Z. Guiberson. It's a great story about the life of a saguaro cactus. We made the anchor chart below, summarizing the story of the cactus hotel. I hope you don't mind my drawing skills, I'm not much of an artist! The kids really enjoyed the book and I encourage you to use it for a summary lesson or even a general lesson on literary nonfiction texts.