+1 to everything Yan says.

Also, I believe Sora brifters are all triple capable, so that will be OK. You will need:

Mountain crankset
Mountain rear derailleur
Triple *road* front derailleur (Tiagra and Sora are fine). Mountain front der's have a different cable pull ratio than your shifters.
New chain
Not completely necessary if you have thighs of steel, but I'd get a 12-32 or 12-34 rear cassette for even lower gearing.