Yes, you would use all three switches for high heat.
The 3rd switch is for high heat. You have to have the low heat switch and high heat switch both on for the high to work.
Mine does the same thing, when I push the third button nothing.
In the instruction booklet it says...switch one just flame effect...switch two is low...switch three is high but only works when low and high switches are turned on together. So for low and flames switches one and two, for high and flames all three switches. Hope this helps.
the first one is power on, the second one is 900 watts of heat, the third one (combined with the second switch on) gives you 1500 watts of heat. Low may do it for a small 100-150 sq foot well insulated room (or with doors closed to keep the heat in). I wouldn't count on this for much more unless your central heat is turned up to 74 degrees (as mine is right now with this heater on)