The lower speeds are accomplished by running the power supply through various resistors that drop the voltage which makes the blower turn at different speeds. The lowest setting uses all the resistors to drop the voltage as much as possible to turn the blower as slow as possible. High speed uses no resistors so the blower gets full power and turns as fast as possible.
Since speed 1 and speed 2 use the speed 3 resistor to drop there voltage and speed three is not working as well my guess is the speed three resistor is burnt out in the device referred to as the blower motor resistor or blower motor speed control module. This is located near the blower motor and is held in place with 2 screws. There will be several wires and possibly two connectors plugged into it.
These are really common failures in many vehicles. The parts are usually available at your local auto parts store and they are typically not that expensive.
Hope that helped.
I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa
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