So I grabbed a deck of cards and had them grab their individual white boards. We started coming up with 3 digit numbers, and practiced away!
The game transformed as we played, growing from just addition, to just subtraction. For a while we played by setting up the problem, then I'd call out "add" or "subtract". Later, I'd let a child choose. (They were pretty tricky... they'd figure out which would be easier or which would have less regrouping needed. I was pretty impressed they were figuring that out in their heads!) After a while, the game evolved into adding AND subtracting the 2 three-digit numbers, and eventually subtracting, then adding to check their work.
The freebie below has the directions and a printable work paper for children to work on alone, with a partner, or small group. The children can work on whiteboards, but the printable is handy to have a record of their work. Click the image or download HERE to download freebie!
If you're interested in more Math Game freebies, see THIS BLOG POST.