Hi, I have 7 hours between flights. Is there a place in the terminal where I can have a 3 hour sleep in Guangzhou airport

Pullman GZ Baiyun Airport Hotel is suggested for you. It is about a two-minute walk from the airport terminal. Or inside the airport, there are lounges for First/Business Class passenger and you can go there to have a rest.
In the terminal, you can easily find the hourly lounges by following the direction boards.