Yes, you could just use 3 packs, but consider this:
According to MrMalty.com a 1.083 OG 5 gallon batch of ale, needs 283 billion cells. So yes, without a starter, you could pitch 3 (actually 2.9 per the site), but with a starter with stir place, you only need 1 pack.
However, you also have to factor in the age of the yeast. Above numbers are with yeast packaged today. If the date on the smack pack was say 4 months old, you would need 12.9 smack packs without a starter vs 4 with...so cost could factor in. And just FYI, if you use White Lab vials instead, the date on the vial is "best by", the actual date is four months earlier.
Saying all that, I did start myself by just using X number of smack packs, but have since moved to a stir plate starter. I find that not only does fermentation start quicker with a starter, it also finishes much quicker also. I also find that I am getting a cleaner, better attenuated final product.
Member of the Metro South Homebrew League (Mash-Holes) - Weymouth, MA
Primary - Amber Ale coming soon.
Secondary - Empty
Kegged - El Hefe Weizen, Patersbier
Bottled - Nothing