RainmanP

There is more to the issue than you realize. You physically cannot put a triple crankset on a bottom bracket sized for a double crank. The length of the spindle for a double is shorter than a triple. Spindles are sized so that the inner chainring barely clears the chain stay. If you put a triple crank on a shorter spindle bb, the little chainring will be jammed into the chainstay. So to go triple you need a bb with a longer spindle. Bottom brackets are not that expensive so this is not a big expense item. You would also need a front der designed for a triple because a double probably does not have enough range of motion. A front der is inexpensive also. Oops, you also need a triple front shifter. So there is a lot more to the whole thing than just a centered chainline. Ask your LBS what they would charge to upgrade the bike. They may even have one set up already or can get one with a triple for only a little more than the double.