I am a third grade teacher in New Jersey. I have a few ideas.. Social Studies- We study communities - Anyone interested in putting together a community project that they would like to share with us. We can tell you about our community and you could tell us about yours. Then we could compare and contrast them- either through a venn diagram, through a compare and contrast essay, or both. Or maybe you have a better idea? I am open to anything that would get the students interested in learning.
Try "The Way We Are" project in ePals. It's free. See the calendar for a clear overview of how to structure this project for success. There are hyperlinks to handouts you can use, lesson plans, extended learning through books and websites, etc.
You can "join" ePals Global Community and then search for other classrooms to collaborate with. The database has 600,000 classrooms in 200 countries. (Lots in US if you just want a grade 3 US class, too.)
You can also get free, safe and protected, ad-free email for your students from ePals. That way, you can do the project, and they can communicate easily and safely.
Someone who has done this for years with students in Virginia schools -- getting rural, urban and suburban grade 3 classrooms to collaborate on Virginia Standards of Learning -- is Adina Popa. She is very active in ePals, as school tech coordinator, to help all her teachers have matches, get videoconference sessions, etc.