The two bolded sentences are contradictory.
If you need to exit the secure area in terminals A & B (first bold) to get on the tram, then it's "before security" for those terminals.
This map implies it is near the security checkpoints for gates 70-129 on level 3.
View attachment 956199
Here's a video that shows the path (jump to about 8:30 into it):
Here's a screen shot of the video. You can see the big C (circled) next to the security for gates 70-129. And where it is in relation to Terminal B (in green in the map above).
View attachment 956209
The Universal Store is just behind the camera and to the right.