Selecting your dial-in based on previous performance isn't enough. There are other constantly changing external factors to take into consideration, such as:
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Compensating for these ever changing factors requires you to understand how they affect you and your car. For example as the air cools toward the end of the day your engine will make more power requiring you dial in a little faster.
In Bracket racing there is a huge discrepancy between the performance levels of any two cars. Many tracks try to minimize this by breaking the bracket racers into classes (eg. 12.00 - 12.99 and 13.00 to 13.99). In order to make a close race out of it, each racer selects a "Dial-In" time. Even if you cut a perfect light, if you can't run close to your dial in you are going to lose! If your car was so consistent that it ran exactly the same ET every run then your job is simple, scribble that number on your window and you'll win a lot of races. In the real world however, no matter how hard you try your ET will vary from run to run. Really consistent cars may vary only a few hundredths but usually you're looking at a tenth or two and that's where the work begins.
Say you dial in a 13.05 and your opponent dials in a 13.25. When the starter flips the switch to start the countdown, your opponents countdown starts .20 seconds sooner than yours (13.25 minus 13.05 equals .20). So he leaves .20 sec sooner than you. Theoretically, if you both have exactly the same reaction times and your cars both run exactly on their dial-in you will cross the finish line at exactly the same time. Well, in reality this will never happen, so whoever runs closest to their dial-in, with the reaction time factored in of course, without breaking out wins.
So let's go back to your three time trial passes. Your times were 13.31, 13.07 and 13.21. What this means is that while your car is fairly consistent, you need to be careful picking your Dial-In. You know your car is capable of at least 13.07 and as you go a few rounds night time will be soon setting in. As the air cools, you can count on your car potentially running a few hundredths quicker. The safe thing to do here is to dial in a 13.05 to minimize the chance of breaking out and if you cut a good light and are ahead of your opponent around the 1000' mark you can always tap the brakes for extra insurance.
As you can see, dialing in takes a lot of time and effort. Tracking all of your runs in a logbook is the best way to know what your car will run under different conditions. If you don't have one, get one (print out ours). Really take the time to understand your capabilities and your car's limitations. Learn to be consistent and you will win races!