Another way of adding more than 2 digits is by grouping numbers together and adding up their answers. Here is an example:
In this example we started by adding the 2+1=3 and the 2+3=5. Then taking the 3 and the 5 and adding those together we come to the top answer which is 8. The children used buttons to start them off and show them that when adding we are putting things together to finally come to our complete answer using all numbers (props) together.