Yes, the third party candidate should be briefed about national security topics. National security is a hot and important issue for America right now and it is necessary for the public to know where each candidate stands in terms of these issues. The third part candidate should be briefed and questioned just as much as the other two to ensure that they have a grasp on what is happening and what needs to happen in order to ensure the security of the country.
Anybody can run for president and therefor get in on the security briefings. This could turn out to be a disaster. I don't believe that anyone should be briefed unless there is an actual chance of them becoming the next president. Someone who is polling in the single digits surely has no business learning that much important information.
No, the third party candidates should not be briefed on national security topics until they reach a certain percentage of favorability in the polls. There must be a legitimate chance of them becoming president for them to receive such information. Otherwise, anyone can run for president and ask to be briefed.
The third party presidential candidates do not need to be briefed on national security topics, because they have no chance of winning the election. No major third party candidate is polling anywhere near the two major party candidates. When the time comes that we have a viable third party candidate with a shot at winning, she can receive national security briefings.