Hi.Just wanted to clarify if I can avail of the 72 transit visa. Here's my itinerary

1. Los Angeles- Beijing (via Air Canada layover in Vancouver on FEB 16
arriving in Beijing on FEB 17 at approx. 4 pm
2. Beijing to Hong Kong (leaving at noon on FEB 19) and buying a separate one way ticket from Beijing to
Hong Kong via HONG KONG AIRLINES OR AIR CHINA
3. HONG KONG TO LAX ON FEB 21 (VIA AIR CANADA WITH LAYOVER IN VANCOUVER)
When do you start counting the 72 hrs and I read that the airlines and tickets purchased don't matter, is that right? Thanks so much for your help.