I am setting up a 3-leg weigh system for a conical bottom cylindrical tank. I...

I am setting up a 3-leg weigh system for a conical bottom cylindrical tank.  I have reviewed the load cell placement documentation, and it presents good information for placement of load cells around the circumference of the tank.

In general, is it better to place the load cells below the tank (CG of tank above load cells) or to have the tank suspended with the CG of the tank below the load cells?

If there is no tipping factor such as wind, pressure, weight distribution due to shape, etc, then it would not make a difference where the load cell are located.  
If there is any influence on the vessel that may cause it to lean or tip because the CG is above the load cells, then it would be better to mount them above the CG.
Normally with vessels, there will be motors, piping, valves, etc. attached that could put pressure on the vessel.  In most cases, it would be better to place the load cells above the CG.