So we know the Model 3 uses the motor/inverter as a heater for the battery. It's very smart to do that, saves extra parts and simplifies the car.
The Lithium batteries in the Model 3 still have the same behavior and requirements regarding temperature. A Lithium battery cannot be charged when it is below 0 Celsius. That's a hard fact. So if you plug in any Tesla and the battery (please note I say battery, not ambient) temperature is below 0 Celsius, the car will halt charging, heat the battery until it goes over 0 C and then start charging. Also, the transition is not on/off. The charge rate depends on the battery temperature. at 0 C or lower, the charge rate is zero. At 25 Celsius the charge rate is full (up to 120 kW). In between the charge rate is reduced analog to the battery temperature.
Tesla knows what they are doing with the battery. Based on my own and other people's observation, Tesla uses active heating for the battery when the battery temperature is below aprox 8 Celsius. If the car is turned on (you are in it, or you start the climate control via the app, or it's charging) the car will heat the battery actively. If the car is parked and off, it will not heat the battery even if it falls below 8 Celsius and even if it is plugged in. Now while this info comes from the Model S/X I assume it works the same for the Model 3 because in the end it uses the same battery type which requires the same temperature management. The only difference is the method the heat is produced. Tesla does just fine with the temperature management. There is no need or advantage to let the owner set the heater by hand.
Charging the car will always also warm up the battery. Charging always has some heat losses inside the battery that will warm it up. If you want to 'manually' warm up the battery, time charging it so it finishes when you want the battery to be warm.