Teensy 3.6 + XBee 868 (Transparent Mode)

The advantage of transparent mode is that you see what you send. This is ok with 2 XBees but with 3 its a crowd. For API example

3.3 volts XBee 868XBee 868 XBee 2CXBee Series2-C


The XBee probably prefers its own power supply.