Convert a compact (double) to a triple front chain ring
In order to change the crankset from double to triple, you will have to replace following things besides triple crankset.


Left brifter to triple from double
Front derailleur to triple
Bottom bracket with longer axle
Rear derailleur with larger capacity if it's short cage


The left brifter (aka STI lever) is the most expensive component. If you don't change the front D, shifting on front is bad if lucky, and the cage may hit the chain. If you don't change the bottom bracket, there is a chance that the inner ring may hit the frame. If it clears the frame, the chainline is bad anyway, so you need a longer crank axle. 

For deciding what rear derailleur to use, you need to add up the number of teeth differences of cassette's largest to smallest, and the teeth difference of front's outer and inner ring, and the total has to be smaller than the capacity that the rear D can handle.

So, you'd be looking at at least a few hundred USD dollars with just components, and if you don't do the work yourself, you'll need to pay for the work which is probably another 100+ USD.