Something to consider: the Sora front shifter will only work properly with a road front derailleur, and most road front derailleurs are designed for larger chainrings and might not work too well with a 42/32/22 setup.

Also, 42-11 is not a very big top gear and you may find yourself wanting more when going downhill. A good road triple is usually 52/42/30 or similar. A lot of touring bikes are set up with 48/36/26 or similar and that provides a good comprimise between the leg-breaking effort required for climbing with a 42 tooth small ring road crank and the frustratingly limited top speed of a 42-11.

You can also just get new chainrings to put on one of your existing cranks, but that is often more expensive than just buying a new crankset. And it may be hard to find replacement rings for an STX crank - I seem to recall they had a funny size bolt pattern.