Either way you choose, stepped starter or multiple vials, it is important to remember that just like your final brew; yeast starters have their preferred pitching rates, too.
If you were, say; brewing a 15-gallon batch with an OG of 1.060, popular yeast calculators will tell you that you'll need a little more than 600 Billion cells. 3 healthy vials by themselves are only going to get you part of the way there.
You'd typically start with a vial in a liter or so of wort, ferment, crash, decant, and put the slurry into about 3 liters or so of fresh wort and ferment again. Assuming a stir plate was used for both of these steps, and the viability was in the high 80's, you'd end up with just over 600 Billion cells - just about right for your 15 gallons of 1.060 wort.
I highly recommend the book "Yeast" by Chris White and Jamil Zainasheff - this book really opened my eyes to the fermentation process and will spell it all out nicely for you.