Identify the sample space of the probability experiment and determine the number of outcomes in the sample space. Randomly choosing a number from the multiples of 3 between 1 and 20 The sample space is (Use a comma to separate answers as needed. Use ascending order.) There are outcome(s) in the sample space.

Step 1 ：The problem is asking to find the sample space of the probability experiment and determine the number of outcomes in the sample space when randomly choosing a number from the multiples of 3 between 1 and 20.

Step 2 ：The multiples of 3 between 1 and 20 are 3, 6, 9, 12, 15, and 18.

Step 3 ：So, the sample space is \{3, 6, 9, 12, 15, 18\}.

Step 4 ：The number of outcomes in the sample space is the number of elements in the sample space.

Step 5 ：Final Answer: The sample space is \{3, 6, 9, 12, 15, 18\}. There are \(\boxed{6}\) outcomes in the sample space.