48x18x21(75g), 48x18x24(90g),48x24x20(100g) 48x24x24(120) Are all great sizes. The best part is you can interchange the lights from one tank to the next and re-use MOST of your equipment for the upgrade if you decide for a smaller size foot print and opt for a bigger tank later down the road. you can even think ahead and build your stand and design your sump according to your eventual upgrade.
I have a 75g pininsula style tank (3 side viewable) And i would have prefered the extra width the 120 would have had to offer. the 18" width makes for a steep rockscape not leaving very much room for coral emplacements. having a 24" width i could have streched out the rock work a bit more and had more room to place corals.
As for the 3 foot tank, well if you were thinking on getting a blue tang (Dory) i would not recomend it as they are very happy and open swimmers that needs lots of swimming space. usually a 90g minimum is recomended. most of the fish you will want you will need a bigger tank for.
Hope this helps
Cheers
