Switching a bike like the Madone to a triple with mountain bike-like gearing in the rear (anything lower than 27t, usually) will not only necessitate a new cassette, but also new brifters (and sometimes new derailleurs). MTB cassettes (11-32, 11-34) are 9 speed and the Madone is set up for a road 10-speed, hence necessitating different shifters if you make that switch--in order to shift properly.

So it is a major deal, labor and parts wise, to make the change (basically a new drivetrain), and don't be surprised if the LBS has to charge you something additional to the bike price to secure both the sale and any kind of profit. That being said, if you really want the low gear options, the time to decide that is when you are purchasing the bike as the shop will be most willing to do what is necessary at minimal extra charge in order to get the sale.

If set up properly, a triple crank shifts as well as a compact (often better, imo), and adds more options at minimal weight penalty.

Margo