In third grade on of the major things we have to do is make sure that we are teaching strategies along with the algorithm when teaching addition and subtraction within 1000. One of the strategies I like to teach is the split strategy.
This strategy involves taking a the numbers and just breaking them up according to their places. It's sometimes called partial sums or even expanded form. If you look at my anchor charts below, you can see how I teach it to my students to help them better understand.
I always begin by teaching with base ten blocks first and then we work into drawing pictures and eventually just the numbers. Sometimes working with regrouping can be a bit of a struggle, so I always make sure we have extra practice in those areas.
I have some freebies to help your class practice addition and subtraction using the split strategy (of course you can just practice the addition and subtraction on the page if you desire!).
To download these awesome freebies, just head over to my blog post Teaching Addition and Subtraction Strategies where you can download them free (and read about how I teach it along with the other strategies I teach!)