Decimal Addition (3 or more decimal places)

The one rule of adding with decimals applies no matter how many decimal places you're working with!

When you add with decimals, you must line up the numbers by the decimals before you add. 

If you don't line the numbers up by the decimals, you'll be adding numbers of different place values together. We all know that if you add 3 dollars and 2 dimes, you don't end up with 5 dollars.  You end up with $3.20.  If you line numbers up by the decimal, you avoid those kinds of mistakes. 

So, no matter how many numbers you have, no matter how many digits come after the decimal point, no matter how uneven the problem looks, line your numbers up by the decimal points, then add.

Example:

$$\eqalign{2.985367+34.5763434443443+19=\\\\2&.985367\\+34&.5763434443443\\+19&\\\hline\text{ }}$$

If it makes it easier, add a decimal to the right of whole numbers (it's there, we just don't always write it out when dealing with whole numbers):

$$\eqalign{2&.985367\\+34&.5763434443443\\+19&.0\\\hline56&.56171044}$$

Finally, you can always check the reasonableness of your answer by estimating: just add the whole numbers.  If you lined up your numbers wrong, chances are your answer will be way off.  In this case: $2+34+19=55$, so we feel pretty comfortable that our answer is correct.