[CAUSA]

I don't think it's necessary to open both legs of a 240 heater, a basic standard 2 pole wall-mount thermostat opens only 1 leg for control then when it's turned all the way down there's sort of a click or detent that opens the other leg. This qualifies it for use as both controller and disconnecting means.

In the OPs case, one of the wires is a constant hot. Another is the element controller and the third one is the fan controller. The reason is that once the element is disconnected, the fan needs to keep running to cool the element down.

X2 on this as well, this is very important for element longevity.

The thermostat should also should be:

Line current design capacity.

Proper heat anticipation function and set up. To prevent temperature over shooting with element cooldown mode.

Should be supplied by the manufacturer but check to see if it can perform the function.