Third graders can learn to spell words by practicing dictation, using worksheets and playing spelling games. The resources at K12Reader.com and FunBrain.com can help parents help their third graders to learn to spell.
The resources at K12Reader.com are designed as a 36-week course, but parents can pick and choose assignments for their child. The website provides dictation sentences, a master spelling list and printable spelling worksheets for third graders.
FunBrain.com offers grade-appropriate games that help children learn, incuding spelling and word games for third graders. Games include word scramble game "Scramble-saurus" and "Spell Check," in which kids must correct the misspelled word.