The 2 speed heater has 3 distinct positions on the fan switch when you move it from one extreme to the other. Off is in the center and low is to the left. "Low-Off-High"
The 3 speed has 4 distinct positions with off being the farthest position to the left. "Off-Low-Mid-High"
I would suggest first checking the fuses. Also there is a resistor in circuit, found on the heater box (near the passenger courtesy light.)