I trim less fat than most people. I leave about 1/2 inch. Sometimes I score the areas of thicker fat. My last brisket was 14 pounds, and I ended up trimming 2 pounds of fat.
I have a #1, so I cut whole briskets in half, with a probe in each half. With a #3 you can probably keep it in one piece, but I would probe both the flat and the point.
The mistake I hear of most with first-time briskets is not cooking them long enough. Either because they underestimate the total cooking/resting time and get impatient/hungry, or they panic when the brisket goes into the "stall". Just remember, somewhere between 160 and 180 the temp will just sit there, or even go down, for
several hours. First-timers want to open the door to look at it, or remove it thinking it's going to overcook. Avoid opening the door. There is nothing you can tell from looking at it. Trust your probe temp (test your probe in boiling water for a 212 degree reading if you are unsure, for peace of mind). I remove the brisket when the flat is 195, and I like the point to go to 200-205. Surprisingly, both pieces seem to reach their target temps at the same time for me, so I don't have to remove them separately.
Example - my last brisket: 14 pound Prime brisket $3.79/lb from Costco. Trimmed 2 pounds of fat. Since it was Prime, I did not brine or inject. Just rub. SI set to 225. Brisket (flat probe) went to 180 degrees in about 2 hours (this is where a first-timer would panic...don't panic). It then went down to around 168, and sat there for about 6 hours (yikes...long stall). Also, don't underestimate how long it can take for that last 5 degrees. Mine took about 2 hours to go from 190 to 195. Total cook time ended up being 12 hours for the two 6 pound hunks. Both pieces were nice and floppy when I removed them. Rested for 3 hours in a cooler wrapped in foil/towel. Super moist. Best brisket ever.
Allow more time than you think to cook the brisket. You can always wrap in 2 layers of foil and a towel, and put in a cooler for several hours. It will only get better. You want to wrap and rest for 1-2 hours minimum anyway.
You should have a pretty delicious brisket using just about any preparation method if you buy either Prime or Choice brisket, and trust your probe temp. These SI smokers really make it easy. "Select" grade brisket can be a little challenging, and would need more attention to prep such as brining or injecting. I would think you can get some pretty good brisket there in Texas though.
