The a/c blower fan on my 2003 f150 works fine on positions 1,2, and 4. Position 3 runs at the same speed as position 2. Just a bad switch, right?

 

It sounds like your blower motor resistor. It is located under the dashboard on the Passenger side. 10 minute job and the part is $20 from what I remember. Did mine about 8 months ago. That said....mine was not working at all at middle speed. It doesnt hurt to change the resistor though. They all go bad eventually. If that doesnt fix it then I would go with the switch.