I also agree that straight sticks are better.
18mm stick are pretty thin, so 15lb-20lbs is probably about the max you will get in there.
I always use 21mm or 23mm casings and can fit 25lbs of stick in the #3. But, I think I use 6-7 racks. I think you could fit them onto 5 racks, but you would have to get the sticks pretty close together. In the pictures below, the smoke casings are 21s and the clear are the 23s. I think we actually prefer the 23s as they stuff better especially if we add high-temp cheese and we can fit more in the smoker. In the end the size difference in the final product from 23s to 21s is pretty small.
Here is a post from one of my previous batches. I have done several since then, but haven't created posts for them as they were pretty much the same and all turned out well.
http://smokinitforums.com/index.php?topic=2284.0