I'm not addressing your wheel suitability here. Let's assume that you're sticking with this bike and these wheels.

To convert to a triple up front, I'd recommend that you look for a used trekking or touring crankset. You will have to change your bottom bracket to match - go with a simple tapered spindle, it'll be most versatile.

You'll want a low gear for climbing under load, 22 or 24 up front and 28 or 32 in the rear, for about a 0.75 ratio.

For the front shifter, your best bet is to keep the Tiagra lever, but remove the shift cable and housing, as well as the left downtube cable stop. Install a simple left downtube shifter, run in friction mode, to control your front derailleur. Simplest, cheapest, most direct. Route the cable appropriately for the downtube shifter, replace the front derailleur to make certain that you have the range that's needed for the triple up front. Any derailleur will do, as long as it fits your braze-on or can clamp onto your seattube.

That's what I'd do, assuming that you want to keep the rest.

Phil G>