Average and standard deviation

Hi All,

I am measuring the height of a tank over time to calculate the flux.
-F is the volume of tank F
-On every time point, the volume is measured 3 times (since the way of measuring is quite fault sensitive)
So F has an average and a standard deviation.
-Flux: (F(t)−F(t−1))/dt, in which dt is the time difference between the time points and A is a constant.

My question is: what is the standard deviation for the flux? The average is just performing the calculation with the average volumes (right?)
AND: the flux is calculated 9 times, how do I get the average and standard deviation of this, since the fluxes calculated per time instance all have their own standard deviation.

The flux can also be calculated by:
in which D and S are the volumes of other tanks. Likewise tank F, measured 3 times per time point.