1984 Trek 560 Upgrade to Triple Crankset
 

I have a 1984 Trek 560 road bike. I've done a little bit of work on it to get it back in riding condition: new tires, new brake cables, greased all of the bearings, new chain, etc. It still has the stock suntour deraillers and downtube shifters.

I've started riding this thing to work. It's only about 5 miles, but most of it is straight up a very steep and very long hill. Even in the lowest gear, I usually have to get off and walk for part of it, and I'm in pretty good shape.

I was thinking about swapping the crankset for a triple to add a few low gears. How hard would this change be? What type of crankset and front derailleur should I get?

It's not too hard at all. You'll need a bottom bracket and a crank. Since you probably have friction shifters, you won't need a new derailer.

You MIGHT need a new FDER, depending on how much "travel" the current one has.

Just recently, I tried to put a triple crank I had laying around on an 85? Diamondback I just acquired.
I could get it to shift to the big ring, but it'd take 2-6 revolutions to make it onto the big ring and was noisy.
Since I still had travel left going to the small ring side, a slightly shorter BB spindle or a "triple" FDER with a bit more travel outward would have solved the problem.
I don't need a triple, but my brother has an identical bike and could use one. Thus I was "experimenting" for him as much as anything.

IF you still have the adjustable cup BB, possibly your spindle isn't "symmetrical". (3-P, 3-S etc. types)
IF so, AND you currently have the "long" side toward the crank, you might be able to get by with "flipping" the spindle. Mine already had the "short" side to the crank, so no joy.

A longer cage rr/der will help too...

+1
I went back to edit and add that info, but my connection went down.

With your current RDER, you'd probably have too much slack in a "properly adjusted" chain when in the granny ring, except maybe the 2-3 largest cogs? You may find that an acceptable compromise if on a budget, only using the granny ring for "bail out" situations.

Your Suntour FDER MIGHT be a HIGH NORMAL. That is, when the cable is not under tension, the default ring is the large one. IF you change to a new FDER, it will be the opposite.

What tooth range do you have on the cassette or freewheel? You may be able to also get away with just adding a bigger sprocket

You should be able to to this without too much trouble. The one concern I would have is that the 560 had pretty short chainstays (~41cm, IIRC) so you may have clearance issues with the chainrings (depending on the sizes you choose) and exacerbated chainline problems in the extreme gearings (large-large and small-small).

If it's stock, it still has a helicomatic in the back, which would make a change of cassette next to impossible. You can make do without a new front derailer, but shifting will be more difficult. Maybe a compact double is the first step, all you need is a crank arm with a 110 bcd. They can be had for fairly cheap, and can give you a much lower gear (42 to 34)