If those three seasons encompass October through April, mostly. But it also depends on the weather, the weight of the flannel and your tolerance for heat and cold. When it's much over 70 degrees, I don't find flannel comfortable. But then there's doeskin, an ever rarer flannel that can be almost tropic weight. So, it depends.
What makes a weight wearable for multiple seasons is the nature of the weave and the thermostat of the individual- someone who either can comfortably wear light weight garments in the cool weather or heavy fabrics in the warm weather. Additionally you must factor in the appearance of the cloth. For example, a few days ago the temperature hit 60 in NYC. If one wore a poplin suit it would have looked wrong. A very fleecy flannel looks wrong in warm weather.
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