All electrical heating appliances give you pretty close to whatever heat their wattage is. So a 250w white bulb will give the same BTUs of heat as a 250w red bulb or a 250w infrared emitter or a 250w anything else electrical.
The difference is "just" price, safety, and where/how the heat is applied.
To my way of thinking it is inefficient to try to heat the whole coop (unless it's a small one) -- you are better off setting up a 'warmth station' where the chickens can get themselves under a lamp or whatever and take advantage of localized warmth. That way you can run less wattage i.e a lower electric bill.
The main application of oil-filled radiators, IMO, would be in very small coops where you *may* want to heat the whole thing, and where in any case it can sometimes be awful hard to rig any OTHER sort of heating appliance in a way that will not be hazardous to chickens or too close to flammable bedding, ceiling, walls.
Is your 3' dimension a width or a height? If it is a height, you should be able to run a normal lightbulb (not even a heat bulb) or a lower wattage IR emitter (like for reptiles) at one end or corner of the coop for the chickens to gravitate towards if they wish. And in such a low space it might make a noticeable dent in overall temperature too. If OTOH the coop is 8x3 and 8' high, you may wish to put in a drop ceiling on the whole thing and especially 'box in' an area over the roost, which again you can run a normal lightbulb or suchlike and just warm *that* area for the chickens to go to when they wish.
Hopefully someone else will wander by and comment on the flat panel heaters, I have no experience with them other than seeing ads.
JMHO, good luck, have fun,
Pat
