I'm with all the other guys. The first rule of barbecue is NO PEEKING and not only because you lose heat and lengthen the cooking process but because you lose humidity and dry the meat out. Small offsets like yours are especially sensitive. A good rule is to keep the cook chamber door closed at all times during the first (and most critical) half of the cook.
More specifically -- when it comes to 3 2 1 ribs, you shouldn't have to mop or spritz at all. Slather, dry rub and 3 hours at 225 uncovered. Remove from the cooker and get the door closed; wrap in foil with a little "mop," beer, juice or other liquid of your taste, and replace in the cooker for 2 hours at 225; open the foil and fold back the edges so the ribs are uncovered on top, but sitting in a foil pan, brush with sauce, and let finish for 1 hour at 225. Remove from the cooker, wrap again in the foil and let rest for at least fifteen minutes. The meat should be very tender -- if not quite falling off the bone.
For "fall off the bone" ribs cook for 3 hours uncovered, foil for 3 hours, and sauce and uncover for 15 minutes only. Remove from the cooker, wrap, and let rest. In other words, the more time the ribs spend wrapped in foil and braising (as opposed to uncovered and barbecuing), the more tender they'll be.
As you barbecue more often and become a little more sophisticated in your tastes, it's very likely you'll find that "fall off the bone" changes to "sack of mush" in your mind. Most of us prefer a little tug to our meat.
The 3 2 1 method requires you to keep a fairly steady 225. You definitely don't want to go below 210 or over 265 in the chamber (generally true for pork), and you'll have to compensate by adding or subtracting time for any major deviation from 225.
The first few months of barbecuing are more about making the basic modifications to the pit, learning to build a steady fire, and learning to keep your nose out of the cooking chamber than anything else.
Here's a link to the basic mods: http://www.homebbq.com/library/SmokerModifications.pdf
The file also has some advice about fire building and charcoal baskets. The file is the "Big Dan" classic and most of the recommendations are gospel. Just my 2 cents, but:
a)Don't worry about lump vs briquette. Use either a good lump (not Cowboy!), or a very good briquette such as Rancher (from Home Depot). Kingsford, Royal Oak (briquette) are adequate but not very good. Low temps and too much ash.
b) Get or make yourself a basket ASAP.
c) Whether basket or free in the firebox, build your fires "Minion Method."
d) Don't worry about moving the thermometer hole. Get yourself a dual-probe, wireless, remote-read digital. Specifically a Maverick ET-73.
FWIW, I don't use the 3 2 1 method, or for that matter foil my ribs at all. But, foiling is the surest method, best for beginners, and you should definitely do it. When you're satisfied that you've got ribs under control, but want better texture, you'll be ready to move on. Next year, maybe.
Small offsets such as yours (mine too!) are very sensitive to the quality, quantity, and time of smoke wood used. For almost everything, it's a good idea to discontinue adding chips or chunk after the first (predicted) half of a cook.
In barbecuing tests for doneness are NEVER made with a clock. A clock will tell you something about yourself (on time -- happy; late -- unhappy, miserable if you're married; early -- ambivalent), but nothing about whether meat is cooked. Even a thermometer isn't much help with ribs, because it's hard to get accurate readings when too close to a bone.
The best tests are visual and tactile. Visually, rib meat should draw back from the ends of the bone, by at least 1/2" on the long side. Meat should tear from the edges and (what's left of the) skirt easily. Bones should have at least a little twist in their sockets. The most revealing rib test is the "bend" or flex test. Using tongs (or asbestos fingers if you're an idiot like me), pick up the slab from the center. When (and not until) the ends point to your shoes the ribs are done.
It's a good idea to plan for an early finish. Whether planned or unplanned, if the meat is cooked early, remove it from the cooker, make sure it's well wrapped (I prefer saran wrap, but since you're working with foil stick with it), and set the ribs aside somewhere draft free (if holding more than a couple of hours, store in a foam cooler packed with newspaper or the kitchen oven, preheated to 200 and thermostat turned to off when the ribs go in-- don't worry about the saran, it will be fine). This resting period is part of the cooking process, it will make the ribs taste juicier. 20 minutes before service, unwrap the ribs, brush with sauce, and return to a hot (250 - 275 if possible) cooker; or unwrap the ribs, and using direct heat, run them over a medium-low grill (lowest gas setting or at least an 8 count with charcoal) for five minutes per side, then sauce and allow another five minutes per side. In either case, you're trying only to warm the ribs through and glaze the sauce -- but not further cook the ribs, and DEFINITELY NOT BURN the sauce.
The grill finish is particularly nice, leaving a glaze with a food-stylist's touch of scorch. Also it will scorch off any membrane you left during prep. But, it's problematic if the ribs are already cooked so tender they want to fall apart if you handle them. Another reason to stop short of "fall off the bone."
Hope this is of some service,