I have a Pulse 21 gas single zone forced air furnace. The blower motor is supposed to have 3 speeds. Right now everything works but when I turn on my AC the blower uses the same speed that it does for heat. Could there be something wrong or do I just need to rewire something?

That is an area I haven't gotten into yet. A 3 speed motor usually has 2 different windings. You have the choice of one, the other, or both for the highest speed. Often the speeds are selected by placement of jumpers on the control board. The different speeds may be used to customize the blower for different duct systems. You should be able to identify the leads to the 2 windings and jumper power to each one and see if they both work. If they both work, then you would need to study the installation manual to determine which wires go where for the best operation. It may be the current set up will best meet your needs.

So the speed setting is a manual setting and the call for the blower motor from the thermostat will always produce the same speed no matter if the heat or ac is on.

More complex furnace controls have jumpers to provide different speeds. The blower on my furnace has a fat bundle of wires going to it with who knows how many speeds. It uses a higher speed for the air conditioning. Since mine doesn't seem to be broke, I haven't worked on it. I am sure in different climates, the furnace and air volume needs vary. More restrictive ducts would also call for higher speeds.

Do you know of any resources to obtain Lennox installation manuals?