Addition and subtraction with exchanges can often be confusing for the children as each calculation involves so many steps. To help you support your child at home, the White Rose videos (with problems) are accessible following the link below. The videos talk the children through each of the steps using pictorial examples.
It is vital that the children have an understanding of the concept of place value (hundreds, tens and ones) using the pictorial method alongside the formal column method (which many parents and teachers were taught when we were at school). Please do not just teach your child the formal column method alone as it does not develop their understanding of place value which is so important in all areas of Maths.
The main addition and subtraction Year 3 objective is for children to add and subtract two three-digit number with exchanging, so these are probably the most useful videos for you/your child to watch and understand: