With box your max dimensions assuming subs up port back double baffle on top you could get around 9 cubes @ 36 slot ported. Maybe a little more if you play around with it.
If you use 3 6" aeros you will have more than enough volume to tune it around 30-32hz. Depends on what you are going for.
Im sure others will chime in.
edit: this was for the 3 15s you can fit either one in your space if you work with it.