Math Games
I recommend math playground and cool math 4 kids if you're looking to play against the computer or another person.
http://www.ixl.com/math/grade-3
http://www.thegreatmartinicompany.com/
http://www.kidport.com/GRADE3/MATH/MATHINDEX.HTM
https://www.mathplayground.com/
http://www.thegreatmartinicompany.com/
http://www.softschools.com/games/
(soft schools games are great for improving concentration and memory skills - try path memory or word scramble)
www.arcademicskillbuilders.com
https://www.coolmath4kids.com/math-games - try the amusement park of math
www.nlvm.usu.edu/en/nav/vlibrary.html - try the virtual activities and manipulatives