\sigma^2{_{X + Y + Z}} &amp;  = \sigma^2{_X} + \sigma^2{_Y} + \sigma^2{_Z}\\\sigma^2{_{X + Y + Z}} &amp; = 6.46^2 + 6.46^2 + 6.46^2\\\sigma^2{_{X + Y + Z}} &amp; \approx 125.1948 \ \ \ \text{and} \ \ \ \sigma \approx \sqrt{125.1948} \approx 11.19