In home automation, a light switch and an outlet are generally treated the same as a “switch” because both have on/off capability. So if you just want on/off, you can look for smart apps which will turn the switch on based on temperature.
you can use the smart app which is in the marketplace section of the official smart things mobile app that is called “virtual thermostat.” You do need to have some sensor that reports the temperature as well as the fan switch.
- Open the SmartThings mobile app
- tap on the market place icon (asterisk at the bottom right of the screen)
- choose “smart apps” at the top of the screen
Four) choose “climate control”
Five) find the smart app and follow its instructions.
There is another one called “whole house fan” in the same section which might also work.
custom code alternatives
If you’re willing to use custom code, you will find several that allow you to control the low medium high options on a fan switch.
If you haven’t used custom code before, first read the overview faq and that will give you an idea of how it works. But it’s pretty straightforward, you are just going to copy someone else’s code and paste it into your own account. 
After you read the FAQ if that sounds like something you would like to try, you can use the quick browse lists in the community created wiki to find custom code for HVAC cases, including fan control.