Imagine a question and stream-of-consciousness explanation for which this is the answer: March 14 , 2016
Question: Principal photography began on March 14, 2016, in Vancouver, British Columbia. Previously it was revealed at San Diego Comic-Con International that filming would begin in February 2016.

When did they start filming the death cure?
Stream-of-consciousness: The relevant information to answer the above question is: Principal photography began on March 14, 2016, in Vancouver, British Columbia.