What does the "3rd swab" refer to in the context of PCR tests for Thailand?

The "3rd swab" refers to a PCR test that was initially required for travelers under certain quarantine conditions when entering Thailand. This was mostly applicable in the context of the original sandbox program, which mandated multiple tests. Travelers seeking refunds for their PCR tests may need to specify which tests they are trying to refund, as one comment indicated that a refund could be obtained for the first test completed.