Trevor, I believe #3's stack is a homemade affair, and has no door to access the inside bottom of the screen. That is why we have to take a maul handle or similar, get up near the top of the stack and beat the screen a few times. When we burn pine with bark on it is when the screen gets easily clogged. Bark cinders get sucked through the flues and get caught in the screen. They don't all get caught however, and at night when she is really working it is pretty spectacular as the sparks and cinders fly through the screen.
On the positive side, those bark and pine (or whatever wood we can obtain) cinders scour the flues. 3 doesn't require as much sanding as our oil burner in my experience.
dan