if you intend to stick to 3ft, why not go for 3x2.5x2.5
quite a nice tank to work with for a start, not quite a problem to reach the bottom of the tank, easy to maintain, probably need a tunze for a good circulation....
the height you have proposed may be a problem, hard to reach the bottom as wat the brothers have said earlier and lighting plus circulation problem too....
would realli advise you to go straight for sump and IOS, canister realli dun work as good as having a sump, coz i have experienced it myself, just upgraded to a sump.....of coz u need the protein skimmer as well, very important i must say...
my 1 cent