what size is your tank, and do you have a heater?
Your goldfish will need 20 gallons each, so if you have 3 you'd want about 60 gallons for them (they grow fast). You'll need heavy duty filters and to do large water changes each week to keep up with the bioload. Goldfish can be hard to keep, so if you don't think you're up to a 20 or more year commitment (yes, they live that long), I advise finding a good home for them with people who know how to care for them.
Your betta will need at least a 2.5 gallon tank, heated to 78-80* F. In a 2.5 gallon tank, the minimum ammount of water changes is 2 50% and 1 100% water changes a week.