The short answer is yes you can use it without a cooling fan.
Having said that, there is a surpring amount of complexity around the topic. If the Pi starts to overheat it will slow down on its own. Overheating is a function of air flow, whether there is an enclosure or hat, how hard it is having to work, etc. There are some good posts on the subject on Element14, eg Raspberry Pi 3 Cooling / Heat Sink Ideas
The short answer is yes you can use it without a cooling fan.
Having said that, there is a surpring amount of complexity around the topic. If the Pi starts to overheat it will slow down on its own. Overheating is a function of air flow, whether there is an enclosure or hat, how hard it is having to work, etc. There are some good posts on the subject on Element14, eg Raspberry Pi 3 Cooling / Heat Sink Ideas
