BBC bitesize video and worksheets introducing column addition.
Subtracting 3 digit numbers together explained clearly, with follow on exercises.
Adding 3 digit numbers together explained clearly, with follow on exercises.
Introducing column based subtraction with 2 digits - very cleanly explained with follow on exercises.
Video on columnar addition - Sal Khan, as always, is fantastic at explaining core maths ideas in a way that is easy to understand.