Why do I have a three flue chimney?

Do you have a furnace, a water heater, or a clothes dryer that uses propane or natural gas? Any of those vents through a flue.

Do you have a furnace, boiler, or water heater?

Or course. Whether you need them or not.

In case of an emergency. If this were a real emergency, and one flue was clogged, your fireplace would be instructed to switch to the other flue.

Start a fire in a fireplace and see where the smoke comes out. Make sure the damper is open beforehand, or the room will fill with smoke. Wrap up some newpaper in the shepe of an ice cream cone, light the top and hold it inside the fireplace as high as you can. This starts the up-current. If you just light kindling or paper on the floor of the fireplace, it may still fill the room and the house with smoke even if the damper is open.

Even if your chimney needs work, you can still have a little 10 minute fire, I think.

Or drop a marble down the opening and see where it comes out.

The reason there are 3 instead of 2 is that they thought it would look nicer. One either doesn't do anthing or it's like your own two nostrils for one windpipe. In the old days, they decorated buildings with gargoyles. Now they use pretty chimneys.

I have two chimneys, just stove-pipe, one for the fireplace, one for the oil furnace. I had them both cleaned early on and then decades later I wanted the furnace one cleaned. The first guy said I needed all new pipes from the furnace to the chimney, about 8 feet with at least 3 turns. They looked fine to me. The second guy said they were fine.

There are ways to make sure your fire makes a lot of smoke. I dont' know what they are. Ask an Indian. or google,

Oh, yeah: "What can I burn to make a lot of smoke? Burning green or softwoods (pine, fire, cypress) can cause a lot of smoke that will make it unpleasant to sit around the fire. Here are our expert tips on choosing the best firewood."

Or just use whatever smoke there is. I'm sure it will be enough.

The flue with the cover is not a dummy flue.

Or, there's always the Bond, James Bond, technique: