Train from HK to Guangzhou flight to a 3rd country

You are not allowed to do that. The only way to enter Guangdong province is by plane. You can exit it in different ways, but thre entry has to be done by air only. In other words, you can fly from Hong Kong to Guangzhou and after that leave mailand China to a third country. If you fly to Guangzhou, you will be eligible for 144 hour visa free transit. 72 hour option was replaced by 144 hour years ago.

By the way, you should be aware that you will have to have a ticket for the third country bought and ready to show already in Hong Kong to prove your transit intention.