Thanks everyone!

And the key term in this discussion is "braise", which means that the meat is not submerged in liquid but surrounded by it, if that makes sense. Brown the meat first with the aromatics (onion, carrot etc.), add the liquid and at a relatively low temperature you can practically just walk away from it for hours.

While thickness counts somewhat when cooking brisket, the most important part of the braising process is to break down the muscle. It needs long, slow cooking. I've cooked a 4 lb brisket for three hours @ 325 F and it was perfect.

Adding on to what DonnyG said, another major factor is thickness. Is 3 three pound brisket the same thickness and width and the 6 pound one, or is it half as thick? Would cook a lot faster then.

Think about a beef tenderloin: it's a long cylinder of meat, with small adjustments to time, a 2 lb roast will cook in about the same time as an 8 pound one, provided the cylinder's about the same diameter.

To truly talk cooking time one would need to also talk about cooking temperature. For a cut like brisket you need to cook using a braising method of cooking which requires longer cooking time at a lower heat to insure that the meat wont be dry. For this The idea is to break down the connective tissue which takes time but also when you braise meat the muscle contracts and expells it's juices and then you have to cook even longer until it finally relaxes and that's when it's ready. I would be looking to cook for longer than 2hrs if you have the time. If using a moist heat I would give it more like 3hrs at 250F- 275F if you have more time then lower for longer and cook until fork tender.

Three pounds would take about 2 hours. Brisket needs to be cooked for a long time.