Here is a list from Lutron, it's on the Caseta site, but Caseta and RA3 use the same app.
surron
https://www.casetawireless.com/us/en/works-with
Lutron Wireless Thermostat, powered by Honeywell®
Provided by Lutron, and backed by trusted Honeywell HVAC technology, for temperature and humidity control in one powerful solution. Model L-HWLV2-WIFI.

Honeywell Wi-Fi thermostats
Choose from a wide variety of thermostat options to control the temperature in your home, from anywhere. Compatible with all Honeywell Wi-Fi thermostats, including Lyric Round and T Series.

ecobee Wi-Fi thermostats
Enjoy the right temperature in the rooms you use the most, with the ecobee3 thermostat and remote sensors. Compatible with the ecobee3 HomeKit-enabled thermostat and all other ecobee Wi-Fi thermostats.

Carrier® Côr™ and Wi-Fi thermostats
Control the temperature in your home from wherever you are with Carrier Wi-Fi thermostats. Compatible with Carrier Côr thermostat and Comfort™ Series Wi-Fi thermostat.
I call tell you from first hand experience, the Honeywell T-9's work great with RA3 (were not an option for RA2), you just need to have the client connect their Lutron account to their Honeywell Home (Resideo) account. These units also have the option to add a wireless remote temp sensor, which is nice.
The Honeywell "Wifi Smart" model also works, but it users an older version of the Honeywell app. These were introduced in mid-2010s if I recall, so would recommend the newer model.
I've also connected RA3 to Ecobee and works very well.