About three bags will give you a depth of three inches. Most folks tend to have a few inches at the front and progressively go deeper as they get to the back.
3 bags will probably do the trick. You're looking for about 2.5 - 3 inch depth. 4 bags will give you the complete 3 inches while 3 bags will give you about a 2.5 inch depth.
If you didn't know, you can purchase your flourite at Petsmart, and pricematch it against www.bigalsonline.com flourite price. Print out the flourite page, and head over to your local Petsmart, pick up 4 bags, and take it to the counter to pricematch it. $15/bag.