Thank you very much for your contact.
Do I understand you correctly, that you already installed a PRTG Probe (I assume Remote Probe?!) on Device B? If this is the case, add a device on Device B with the address of Device C. Afterwards, you can simply deploy a Ping sensor to monitor the connection between Device B and Device C.
If you don't have a (Remote) Probe on Device B, please check out the WMI Remote Ping sensor instead. The sensor remotely connects to a Windows system (Device B) using Windows Management Instrumentation (WMI) and performs an Internet Control Message Protocol (ICMP) echo request ("Ping") from this device to a specified target (Device C).