I have three base board heaters all 240V each one has a dial on the side acting as a t-stat
What the customer would like is for all three to work together with one wall mounted thermostat please explain how you guys would wire this.

Thanks

Inside each heater, disconnect the internal thermostat, and connect the incoming power lines directly to the heating element. Leave the overtemp cutout intact!

Feed the 3 heaters through the new thermostat, using a relay or contactor if the combined heater rating exceeds the contact rating of the thermostat.