Perhaps, a few things have changed since I was in the Corps. I believe we use to only have 1min. to do the situps, and 80 was a perfect score, 18min. in the 3-mile run, and the same for pullups, 20.
Situps: nothing like practicing the actual test. Doing sets of 12-15 reps will help, but it's not the same as doing 100. So the best advice I can give you is to practice doing high reps, with someone holding your feet. Make sure they hold them tightly, especially during the test, even if they have to kneel on your feet.
A firm foundation will help you to easily crank out about 1 per second. And with that in mind, pace yourself, i.e., don't crank out 50 in 30 seconds, but create a cadence of 1 per second, and stick to it. One last thing, keep your butt as far away from your feet as is comfortable. This will give you better leverage, and make the movement easier.
Pullups: They used to allow a slight "kip" out of the bottom. If so, LEARN IT, and you will do 20 with very little effort. Think of it as the differnce between a Military Press, and a Push Press. Just that little bit of momentum generated at the start will greatly enhance your performance.
If you don't know what it is, taking an underhand grip, or overhand, if you prefer, you basically just pull with your arms, lean back slightly, and lift up your knees about 1 foot, while swinging them slightly forward, at the bottom of the movement, then explode to the top while straightening your legs. Easier to demonstrate, than describe.
Basically, the object is to eliminate most of the weight of your lower body, since your legs are moving downward, while your upper body is moving upwards. But you really have to explode out of the bottom, and sorta coast after about half way up...the momentum will get you there. But if you want to tough it out, practice the movement..!
In either case, practice, practice, practice. Don't worry about curls, etc. Practice doing pullups.
3-mile run: You didn't say much about this. Fortunately, I was a all-conference, all-state long-distance runner in high school, so it wasn't a problem for me. There was hardly ever a time while in the Corps that I didn't max the PFT. What can I say, run....
Good Luck, and Semper Fi..!