The Super 3T is the entry into the Omega line and is best on a bookshelf, credenza, or stands. They will also work on a desktop if you have the real estate to accommodate the wide baffle.
The Super 3 Desktop is the smallest Omega ever made and has a narrower baffle making them suitable for beside a computer monitor. The downfiring port gives them more flexabilty on the desktop being they don't require a wall behind for bass reinforcement - they use the desktop.
The Super 3i is the Leatherman of the Omega line. Great on a larger desktop, credenza, or stands. The fullest and best sounding of the three.
They all sound fantastic but the 3i has the edge. Bass wise the 3i is the best, then the 3T, then the 3 Desktop. The 3i and 3T often don't need a sub (depending on situation), and even though the 3 Desktop sounds full, a sub is a nice addition. The 3 Desktop with a sub will however put it in a different class than the other two by themselves though. With the spikes and downfiring port the 3 Desktop can also be a true bookshelf speaker with books on either side and touching the wall behind. Hope this helps.