Re: How to add separator (comma) inside a formula?
Actually no difference, for me it was faster enter the formula ans copy/paste it here. In general, for whole numbers you may use something like TEXT(A1,"#,#"). Another story if you would like to make different formatting for positive and negative numbers, add text and or colors into formatting, etc - all that additional symbols matter.
In particular, you may format your cell with =SUM(D3:D12) as "Total: "#,### (enter this into the Custom format of the cell) that gives the same result.
Comma just separates your number on thousans, millions, billions, etc.