We pull from many sources.

I am not sure if you are going to teach from a Biblical perspective, but even if you do not, using the 6 days of creation as a general guideline works real well.

Look at what was created each day; then go to the library, or use Usborne books, they are often Internet linked so they can provide many additional information on top of the book.

For the 3Th grade you can also do unit studies based on the Magic School Bus, with the books, DVDs, and computer software you can go from space to the inner place ^_^ (human body).

Most libraries carry all of them, or go to the Scholastics web site and buy them at a discounted price.

If you are more interested in a structured curriculum try either Apologia, or Christian Liberty Press, they have some real good early childhood science materials.

We did not start using Apologia until they were about 13, or 14; beginning with high school level general science, and used it as an independent study book.

http://ebiz.netopia.com/clpress/science1/

We used God’s Wonderful Works as our guide line book, we did not use it as a work book.

http://www.christianbook.com/Christian/Books/easy_find?category=Homeschool&N=1030470&Ne=1000000&Nso=1&Nu=product%2Eendeca%5Frollup&format=1014667&Ns=product%2Enumber%5Fsold

Apologia at a discounted price.

http://wwwrocksolidinc.com/ is another home school store that sells much of the materials at a decent price.

Hope this helps a bit.

http://gravitaspublications.com/

http://www.hometrainingtools.com/catalog/science-curriculum/cat_real-science-4-kids.html

http://www.theoldhomeschoolhouse.com/Homeschool_Reviews/reviews.php?rid=916

http://www.arn.org/realscience/realscience.html