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In smokers 36 inches in length or less we go a minimum of 4 inches, and prefer to go 5. At three inches you will have some issue's with having too high of a radiant heat from that plate on the firebox end. You can offset that a little by going with a thicker RF plate. 1/4 inch plate will help a little. I will put this in a little bit of perspective. At 4 inches we can grill chicken on the firebox side of the smoker while running at 230°. This is using 3/16 steel for the RF plate.