HELP! I need to add 3 sigma to my bell curve.

Assuming that you want to add a vertical line at the point that you have calculated to be 3 Sigma, cell C12 on the 'Fire' sheet, the following will work:
1) Arrange a table as shown in B19:C21, note the the values of B20:B21 are linked to C12 and the value of C21 is linked to F6
2) Select and copy the table,
3) Select the graph and select paste special,
4) Select the New Series, Categories in First Column, and Series Names in First Row options
Let us know if you have any questions.