Greg Tang has a website full of great resources for math for young children. Here are a few games that might be great for third graders.
NumTanga is a matching game that helps builds students’ number sense and fluency with numbers. It can start from really basic work with numbers and grow up into fractions and more.
Math Limbo is another game that helps build students’ fluency with numbers. Students can choose one number or put numbers together to create an answer.
Kakooma is a game that practices addition and multiplication skills.
Numskill is a challenging game. It requires students to think about numbers and about patterns in different ways.
Satisfraction is like Numskill but is played with fractions.