3 weeks should be enough time. wyeast liquid plus a packet of dry yeast should be enough for a wort of that gravity. 1.026 isn't a bad gravity for a sweet stout, and it's awfully close to your target. you're at 71% attenuation which is the low end of the range for 1084, and i don't think you (anyone) can do much better with all the extract. what temp did you ferment at? if you really want to give it a chance at dropping those few extra points leave it in primary, give it a rouse, and bump the temp up to 70-72. you won't do anything negative this late in the game, but i'd be pretty happy with it as is, IMO.