What is the probability of obtaining a "3" on one roll of a die?
Assuming an "honest" 6-sided die the answer as Syamini says is "1/6".
If all possible outcomes are equally likely, the probability of a particular outcome (in your case, "obtaining a 3") is the number of ways of getting the particular outcome divided by the total number of possible outcomes.
If you roll an unbiased die there are 6 total possible outcomes: 1, 2, 3, 4, 5, and 6. The particular outcome you are interested in, a 3, happens only 1 way. Therefore the probability is
If you had asked for the probability of getting a "3 or less" then the total number of possible outcomes remains the same, but there are 3 ways of getting the particular outcome (1, 2, or 3) so the probability of getting a "3 or less" would be