For each question, there are two options: True or False. Thus, if you guess on all of the questions, there is a 1/2 chance that you get the answer right.
If there are three questions on the test, the chance that you get a perfect score is:
1/2 * 1/2 * 1/2
When this is simplified, you get:
1/4 * 1/2
OR:
1/8
Therefore, the probability that you will get a perfect score is 1/8 or 12.5%