first post? Welcome, The_Doughboy!
FWIW I've seen a
1lb disposable tank run my dad's '06 Performer for 3-4 starts (coals sitting in the holders), and it was a
good half-empty to begin with. On the last one we ran it for about 15 minutes or so, but I think it's spent now ... YES I can see the value and efficiency of using a chimney there. Nothing really gets the coals going like a chimney, and AFAIK Weber's chimneys are the best.
I'm not familiar with a "3lb" tank and I don't think Google is, either? The first (i.e. "stainless steel" [ss]) Performers came with a 5lb tank, still available but not with the FLAT side that mates perfectly to the grill's mount. The current 5lb tanks have a curve all the way around but fit "OK" on the SS Performers--I think.
Today I just saw on eBay re-certified original Manchester 5lb tanks with the flat side.
http://www.ebay.com/itm/Reconditioned-5lb-Propane-Cylinder-BBQ-Mr-Heater-Fishing-Tank-OPD-Valve-/111310034310 No bargain per se, but not a total rip-off either. (I need one for mine; I'm on the fence.)
The other "slim" tank (so that the valve lines up with the opening up top) is the 11lb size but might take some rigging. The ubiquitous 20lb tank would have to sit on the bottom grate.
My apologies if this is stuff you're already aware of.