My first caution is that the 3,2,1 has some flaws, mostly in that it adds up to 6 hours, which is about 2 hours too much. Granted, the 2 hours in the foil will keep them moist to a fault in my opinion. 2 hours in foil = rubbery ribs, and then when you try to put them back on the grill to cook in the sauce, they fall apart making it nearly impossible to get that last 1 hour in. If you're bent on doing the 3,2,1 type cook, I reconmend 2,1,1
My next question is why would you want to roll them? Ribs fit fine on the 18" here's a photo of ribs going on my 18" recently
My reconmendation is to only plan on cooking your ribs for 4 maybe 4.5 hours, don't foil them at all. Baste them at the 1,2,3 hour mark, I use a apple juice brown sugar worchestershire sauce cocktail as my baste. If the meat is pulling back at the 3 hour mark, skip the baste and add the sauce. What I plan to do on my next rib cook is cook them meat down for the first hour and the meat up for the remainder of the cook. This is to avoid "over cooking" the bone side
Whatever you choose to do, enjoy your cook and ribs