How do I make a bulk holding tank for fire trucks to fill, using 3 different tanks of different sizes and different elevations?
You appear to be setting this up to fail. Don't do that.
If you have three tanks at different elevations, your fire truck fill point is the bottom of the lowest one.
Your system fill point and vent is the top of the top one.
If the two lower tanks won't hold pressure (typical concrete tank that does not really have a tight seal at the top) you'll need float valves on the pipes that fill them from the higher tank so the water does not just leak out the top.