Can I use a Canon/Yongnuo YN-E3-RT radio controller to fire Flashpoint strobes? I don't need TTL, just manual control and firing.
No, you can not.
The Yongnuo YN-E3-RT and the Canon ST-E3-RT it imitates uses Canon's wireless radio protocol to communicate with 'RT' receivers.
The Flashpoint system is made for Adorama by Godox. It uses the Godox wireless radio protocol(s) to communicate with Godox, Flashpoint, and several other nameplates made for specific sellers by Godox.
Although each system is communicating information from the camera to the flash, each system uses a different "language" to communicate the same information.