BS, I also suggest trying the Tiagra FD with whichever crankset you eventually choose as you already have it. While a Shimano road group triple crankset is assured to work with the Sora/Tiagra combo, I'm not positive the Shimano mountain/trekking crankset's chainring spacing is the same. You should be fine with the RD.

Chances are high that when changing cranksets from a double to a triple, a BB with a longer spindle will be required, again this is a test fit item. The Touring Forum is also a good place to ask questions as many there have been through the mix and match drivetrain/shifter builds.

Brad