Addition of two 3-digit numbers is usually done on paper, using the following standard method.
The method is to add the units first, put the units in the answer, and ‘carry’ the ten into the tens column. Then add the tens and continue in the same way into the hundreds. Whilst all the questions on the free maths worksheet have carrying to both tens and hundreds, this will not always be necessary if the sum of the digits is less than 10.
To see each step in more detail, together with a page of questions and answers, view the pdf below:
Standard written addition of two 3-digit numbers
There are also lots more written addition pages in our Four Rules section.