I suspect that you have entries in the table that do not contain numeric values but instead contain NULL's. This is a special case in SQL. For instance, 1+0 = 1, but 1+NULL = NULL (or undefined). If the NULL values are to be interpreted as 0 then you must explicitly state this in your query using the IFNULL function.
Also looking at your query you are calling the resulting rows with names that appear to belong to the different fields in the table. So it is not clear whether you are looking to reuse the previously calculated num3 i.e. (num1 + num2) or the value of the num3 field in the table. SQL will always use the field name and never the alias.