So-called "3D" pop is an interesting thing. It would seem to mean different things to different people, and as such can involve a number of different aspects and properties.
In your case (and I believe it's a valid one), subject isolation through creative use of depth of field appears to be the key. I think this is most evident when the subject, and perhaps other foreground elements, are in focus, while the background is considerably out-of-focus. Better still, if there are few if any elements showing a transition between the two - as in the following test shot, taken with a Vivitar 85mm f/1.8 preset T-mount lens. Not a great photo, but it shows a central flower and some foreground floral details in the lower right, both in sharp focus, with a blurred background and very little transitional matter: