Whilst the Primary Framework for Mathematics has little to say about addition of whole numbers in Year 6 it is still worth revising skills and developing fast technique, especially with mental arithmetic.
Here we have a consolidation page of adding three 2-digit numbers, using all the techniques developed so far.If children find this difficult it could be that they are unsure of their basic number bonds, in other words how quickly can they add two single digit numbers. This should be almost instantaneous, if not it might be worth practising over the summer holiday!!