The 1/3 of a decimal is 0.33. Take note that this is already in its shortened form because if you would actually try to make it into a decimal, you will get something like 0.333… which, of course, you would need to shorten. Since the next number beside the second 3 is also 3, this will not be enough to increase the number to 4, which is why the answer is still 0.33.
If 1/3 will be made into a percentage, you will get 33.333%, which of course, you would need to shorten again if you would like to have a clearer answer. The shortened form will be 33.33%. Changing fractions into decimals may be complicated in the beginning, but the more that you know, the better that you will be able to do it.