Do a forum search next time, this topic has been covered about 40,000 times, which is why no one is responding. Water cooler containers are typically OK to use as fermenters as long as you are careful to make sure your wort is always at or below pitching temps (70-80F) when you transfer it.
Just note that due to blowoff, you can realistically expect to fit about 2-2.25 gallons max in a 3 gallon carboy.
Good luck!