If it were my bike I would probably start by changing the crank to something with smaller chainrings... the current crank probably has a 130mm bolt-circle-diameter and these can only accept a slightly smaller chainring, if any.
A basic MTB crankset has a 94mm bolt circle diameter (or 110 mm if you shop around for old stock)and generally come equipped with 22-32-42 chainrings, which can be changed to larger. Changing the crank probably also means you will need a different length of bottom bracket spindle as well. Both these changes can likely be done for around ~$100 - 150 or so.
This change might be all you need to do, but you might also have to change your front derailleur to a triple-chainring specific one (probably about ~20 for a decent quality FD).
As others have said, consider getting a new used triple-chainring road or touring bike. This is likely to be much simpler and not too much more money.