Hello, and thanks for your question.
You can join TSA Precheck, free of charge, if you are a DoD member; you can find all the details, here:
Since you're leaving in three days, I don't know if you have enough time to enroll in the system, but if you go to an Enrollment Center near you as a walk-in, you might have to wait longer (if you don't have an appointment), but it's worth a try.
If you have any further questions, you can contact the Travel Assistance Center at 1-***-***-****. They're open 24/7.
I hope this helps and wish you a wonderful trip.
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