You'll want to base your tank ideas on the stability of your foundation, and the budget you have. The bigger the better when it comes to tanks. For 3 fully grown female RESs, you'll need a MINIMUM of a 200 gallon tank. A 200 gallon tank, after adding equipment, will weight close to 2,000 pounds. This is an incredible weight to put on an average household floor, so you'll want to make sure the floor is either reinforced, or in direct support of solid ground.
The dimensions are really up to you, especially if your considering a custom made tank. I believe most generic tanks over 90 gallons are 6-8 feet long, 15-24 inches deep, and 20-32 inches high.
I recommend reading this article on floor stability in an average home:
http://badmanstropicalfish.com/articles/article28.html
