Can I add a light switch that overrides a set of existing switches?
Your best case scenario is that you have one hot wire that comes into your switch box and is chained to one screw of all three dimmers.  Maybe the wire comes in and is spliced into three wires that each go to a switch, or a clever person has just stripped insulation from the middle of the wire and chained it from switch to switch.  The other wire going to each dimmer would then go out to each row of lights.



image from https://www.do-it-yourself-help.com/wiring-two-outlets-one-box.html

In this picture, the source coming in from the bottom is what I'm referring to. You would have three switches and lights, but the diagram only shows two. If you put a new switch on the incoming hot wire before it goes to the other dimmers (before the black wire nut in the bottom left), you will be able to cut power to everything with that one switch.

Of course that is just one way to wire the lights.  You could also have a switch loop that comes from each row, or a combination of switch loops which would make this a more complicated endeavor.