It may actually be a bad thermostat, and not the actual temp sensor. This is a very common problem when the thermostat isn’t working correctly and allowing the engine to warm up at a certain rate. The way the computer reads the temperature, it is timed. If the temperature isn’t change at the rate it’s supposed to, it’ll show three bars like you see on your display. You can try the coolant temperature sensor first, which is what a lot of people do, with no change. But it’s more than likely a bad thermostat. This is common on a lot of the GM cars of this era.