first batch, not quite 5 gallons

So I brewed my first batch, and I'm guessing I boiled off quite a bit, down to maybe 3-4 gallons. I started with 5 gallons of water, and didn't add any when I transferred it to the carboy. It's been about 3 weeks so I'm heading to the bottling stage, but I have no idea how much sugar to add, every measurement I find is for 5 gallons, and I don't know if less water affected the yeast or anything.

Been reading other posts, so I know it's not ruined, and I'm not an idiot, just want to start the next batch and taste this one. Thanks.

If you have three gallons you would need 60% of the amount you would use for five gallons. For four gallons you would need 80%,

You can use a handy carbonation calculator online.

This one has different styles in the calculator
http://hbd.org/cgi-bin/recipator/recipator/carbonation.html