The version of i2c_t3 included in Teensyduino 1.25 appears to be broken with the Teensy 3.0.
C:\Program Files (x86)\Arduino\hardware\teensy\avr\libraries\i2c_t3\i2c_t3.cpp: In static member function 'static uint8_t i2c_t3::setOpMode_(i2cStruct*, uint8_t, i2c_op_mode)':
C:\Program Files...