The Apple TV has a connection limit of two Bluetooth devices (three including the remote).
This is not something we can work around in FulGaz, as it is a hardware feature. Any Bluetooth devices that are broadcasting data will show up on the FulGaz connections screen for Apple TV, and the Apple TV will let you connect to them, but in-app the Apple TV will only read data from two of them.
This is a connection limit, not a limit to the number of signals the Apple TV can view.
Most modern smart trainers send all their data (trainer control, power, and cadence) over one channel, leaving the second one open for a heart rate monitor so this is not an issue, but if you then wish to connect a third sensor, the Apple TV will have hit it's limit.
For example if you connect your smart trainer, a HR monitor and also a CORE body temperature sensor to FulGaz on Apple TV, one of those sensors will not display data. Or if you have an older trainer, a heart rate monitor, and also a separate cadence sensor, the Apple TV will only let FulGaz read data from two of those devices.