You do this job long enough and you forget that it all looks so much simpler from the outside looking in!
Will Entrekin writes in "Is there ever a time a writer isn't trying to get a three-book deal?"
And the answer is when the economy is good.
Say you're a first time author. You've finished your first 300 page novel and your agent gets you a deal - an OK deal, but nothing quit-your-day-job life-changing. Publishers often want to sign you up for your next novel - often referred to as "untitled" - so they can lock you in at the first-time writer rate. They often "basket" or "joint account" the two-book or three-book deal, so that you have to earn out the entire two or three-book advance before they start paying you any royalties. It's a win/win situation for them. They get the great new voice at a discount rate and if you hand in a "sophomore slump," they can turn it down and ask for the money back.
With a three-book deal, even if your book does moderately well (block-busters are an exception), you won't see any added revenue until three years after the first book has pubbed (because it takes most companies at least one year, but often 18 months, to go from manuscript to print).
So, when the economy is good, you and your agent would expect that your first book would do well and that you should wait for domestic and foreign and film sales to come in to see if you can get a larger advance for your second book, based on performance. You might not, but they should offer you at least what they did for the first one (unless it bombs, which does happen) and the money won't be joint-accounted.
Sometimes the author wants the guarantee of future publishing and writing income (and wants to quit that day job), so s/he takes the three-book deal. Even with a three book deal that covers three years, the author usually has to take another simultaneous three-book deal (or three separate book deals) to make ends meet as a writer.
When the book is part of a series, it's often advised to go for the multiple book deal, too. It's really hard to move a series from one publisher to another, unless it's really popular - and you loose the backlist upsell. But, in a good economy, you're agent would probably be able to get them to drop the joint accounting. Not so easy now.
But when the economy is bad, publishers can just stop handing out contracts and a lot of working writers have no money coming in.
So that's why I say take the three book deal now.