Divide all ingredients by 3 of course. But then also:
1) Assume your boiloff rate will be about 50%. Seriously. Sparge enough to produce an extra gallon of wort, which will boil off if you use the same equipment. If you can use a smaller kettle on your stove, boiloff might only be about 35%. But it will be quite high either way, compared to a "normal" batch size. And along with this...
2) You should see your efficiency skyrocket, since you can sparge so much more.
If not sparging, then nevermind about efficiency, but you will still need that extra gallon of water to boil off. Alternatively, I suppose you could shorten the boil to only 15 minutes or something like that instead of boiling a full hour, but this might not give you exactly the same result, might lead to more haze, and might require adjusting hops up to get the right IBU level.
If you fiddle with your software enough (you're using BeerSmith or some kind of software, right?), you should be able to get all the parameters where you want them to match the original specs.