Electrical – using a light switch for electric baseboard heater

1st, the white wires should be completely covered with black tape. Try putting both same colors at the top and at the bottom, if you have verified all these wires are the travelers from 2, 3-way switches.

In this diagram the light is off. Toggling any of the switches will turn it on.

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First, are you really sure they are still powered? Heaters have a fair amount of "thermal inertia" - they take awhile to warm up, and they also give off heat for some minutes after they're shut off. Obviously, the sure way to tell is to pop the service covers off and test with preferably non-contact testers, ideally a clamp-on ammeter. Although at some point you'll want to measure actual voltage.

One way to measure is your house's electric meter, as it has a spinning disc or similar digital readout that will show rate of power consumption. A stopwatch and a bit of research will tell you exactly what your whole house is drawing. To measure the heater, shut off all other large loads.

Here's a little secret about resistive heaters that I use intentionally sometimes. If you apply 120V to a 240V heater, you get 25% of the heat. So, if you're still getting (roughly 1/4) of the heat an hour after you "shut the thermostat off", there's a good chance a heater is seeing 120V instead of 240V. The very serious question is WHY? Because whatever's doing that could burn down your house.