hey hi Tench1
3x3x2.5 ==> very huge tank!
the issue you will face is
1) difficult to scape. unless you have long long hands or if you get Iwarna to scape and epoxy it for you.
this is a 5x2x2.5 tank. not sure if you can picture how deep it is.
2) unable to reach the mid point of the tank. unless you plan to scape it in a way that the rocks are closer to the tank wall on the left and right side and hollow in the middle..... that might work.
3) you will need to take into consideration the lighting, wavemakers as well for such a tank.
but nothing is not possible bro. I have met a number of reefers in LFS that owns a 3 feet cube! so if they can do it, then surely others can too. But do talk to some of the reputable LFS and ask for their opinions. If you can set it up, I am sure it will be an awesome tank.
Personally for nanos, I prefer cube tanks. For big tanks, I would prefer a longish wide tank. perhaps a 6x2x2 or a 6x 2.5 x2.
I like the panorama look of a long tank... better still 7 or 8 feet.....