This was such an easy math counting idea to put together and my son absolutely loved it. Maybe more than he should have for a 3 year old! I love that it also worked his fine motor skills. Picking up tiny beans can be tricky work!
Kiddo is totally obsessed with numbers and counting. I don't actually know what the average 3 year old's knowledge of numbers is but Kiddo can easily count into the hundreds. How it happened, who knows; I don't remember actively teaching the numbers but I must have, somehow.
First we drew six circles. I didn't care if they were perfect or in a straight line or any of that. The important part is to get to the counting part as quickly as possible. I asked Kiddo for a few numbers and he choose 1-6 (he's very organized). Then I explained Kiddo could put the same number of beans in the circle as the number. He got extremely excited at this idea. Could. Not. Wait.
After he finished, I drew more circles and encouraged Kiddo to choose different numbers. We've done a similar counting with pom poms activity. Or, if your child is ready to count too 100, you could try a a similar activity with popcorn.
You could easily modify this to teach the concepts of less and more or simple addition. We stuck with simple counting for the day.