5X3 is going to be a lot of weight, but 3X3 is NOT too thin. In fact, it is probably overkill for the supporting columns.
Some numbers: A 2X4 beam, 4 ft free span(4 ft between supports) can typically support 450-600 lbs under very conservative requirements (minimum deflection or bending). The tables I typically use(I build furniture and do deck construction) don't have 3X3 but for a 3X4, 4 ft free span, typical conservative requirements yield 750-1000 lbs. A 4X4, 4ft free span can support 1000-1500 lbs.
Post: For post, there are 2 considerations, compression of the beam (crushing it from top to bottom) and deflection or bowing (beam bows out to the side). In this case, the tables used L/D where L = length of beam and D = diameter of narrowest cross-sectional dimension of the post....3 inches in your case. L will be roughly how tall your stand is. If your stand was 36 inches, your L/D value will be 12. Under these conditions, a typical construction wood (some kind of pine generally) the post could support 800+ pounds, again using very conservative values.
How many gallons is your tank? This value is needed to determine the total weight. Once you have the weight, you can then use the footprint dimension(length and width of the base) and come up with a design. I am guessing your tank is something over 180 GAL (A typical 180 is 5ft X 2ft X 2ft ROUGHLY). A full 180 weighs around 2100 lbs. Assuming the same depth and adding another foot of width, your tank is going to be over 3000 lbs. Given your wood dimension (3X3) and the footprint dimensions, I would probably have a post at each corner (duh!) and then 2 in between the corners on the front and 2 in between the corners on the back. Something like this:
X X X X
X X X X
Your beams are going to run across the tops of those. So, your unsupported span is going to be 5ft/3 or roughly 1.6 ft. In theory, you could get by with only 1 column in between for a 3 ft unsupported span, but with the 3X3s, I wouldn't do that myself. What I would do, if I was building it, would be to use 3X3s for the post and then get 2X6 for the beams. A typical 2X6, on a 4ft free span, can support between 1100 and 1500 lbs, conservatively (note that all of my numbers are very, very conservative... i.e. a 2X6 on a 4ft free span can actually support up to 2500 lbs, while only deflecting .1 inches. Given this, you could in theory, have a post at each corner with 2X6 beams along the edges and it would support your tank. However, for myself, I would NOT want to go that route. Anyway, I would get 2X6 beams, 3X3 post and then have a post at each corner, one in between on the front, one in between on the back. Something like this:
X X X
X X X
I would feel completely at ease having such setup in my house with no fear of failure.
Also, I would put in some stabilizing members.... cross members from front to back on both the top and the bottom. Maybe some diagonal braces on the sides running from the top of one post to the bottom of the next. All of these out of something like 1X4 or 1X6. These are NOT to support the total tank weight but just to provide extra stability from shear on the stand.
Hope all this helps.
/www.geocities.com/elgecko1989/Woodworking.html
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I know it's nit-picky, but I couldn't help myself. What you've got listed as your top view is actually your bottom view, which all comes out to the same thing regardless. An easy way to think about it is to picture your assembly or part in a bowl. Your front view will be sitting right at the bottom. Now roll the part up the bowl straight back and you've got your top view. Go back to your front view with the part sitting in the bottom of the bowl and roll your part to either side and you'll have both your side views. Do the same for the bottom view and when you do your drawings, place them in that order, working from your front view, top view is above it, bottom view is below it. Sorry, I'm just bored here at work and needed a break. 
Ah, you just made my new day CFC. I needed that today. Happy holidays everyone.
