The real problem with a stack of 3 barrels is the weight. Even using plastic barrels you're still looking at 75lbs. I wouldn't try stitching 3 barrels together, no way - the rotor would be too heavy and would want to flex - I think it would be better to make 3 separate rotors and put them in a 'Y' config. around the pole or something like that. I'd still count that as one windmill (same tower/pole) and the output of the 3 (once combined) should be pretty decent - whether Volvo would count it for his contest is a different matter.

I've read a claim (can't remember to whom it belongs) of someone getting 50-60w from a steel, double barrel vawt. If that's true then switching from 2 steel barrels to 2 plastic barrels (half the weight, or close) ought to get you some nice gains. Probably not enough to hit the magic 100w mark, though you might get close if you're good. Stitching 2 barrels together is still 50lbs but that seems way more do-able (sane) than stitching 3 together.

I'm really surprised by the low output, CmeBREW...I figured it'd put out 20w no problem and 50w in a hearty gust. I would look at the weight and shed as much as you can. Then I'd start thinking about the best combo of blades, angles, diameter and all that. Maybe acrylic plates would work in place of the plywood, I dunno, but that's the kind of direction I'd be thinking in.

I'm pretty satisfied with the mini squirrel cage I built (1/4" plywood plates, 9" diameter, 2.88sqft) but with the load I put on it, it won't turn in anything less than 15mph and doesn't hit its stride until 30mph....and it's so small I'd be lucky to see 5w when it's spinning full throttle.