Each year as part of my schooling preparation I reread The Well-Trained Mind (aff link), by Susan Wise Bauer. It helps me to do an overview. A lay of the land, so to speak, before I amend and adjust for our purposes. For Third Grade, these are the suggestions. I put the most recommended source under certain topics but, as always in homeschooling, there are many other options and the choice is yours. The pdf is linked below.