Once children are really happy with adding two 1-digit numbers then they can move on to adding three small numbers mentally. There are various strategies that they can use to help them.
Probably the best strategy of all is to start with the largest number, and this should be the case for the majority of questions. Sometimes they might choose to start by adding two other numbers because they already know the answer and then count on the third number. Another strategy to use is to look for pairs of numbers which make 10.
This set of worksheets is found in our Year 2 Addition sectionwhich is developing quickly.