Can the TMC4671 be used to control 3-phase stepper motors?
The TMC4671 is capable of controlling a 3-phase stepper motor, much in the same way that it would control a 3-phase BLDC motor.
The TMC4671 can control a 3-phase stepper motor in both closed-loop (FOC), as well as open-loop. 
This being said, open-loop operation is limited to velocity mode only. Positioning in open-loop is not possible with the TMC4671.
Please also note that the TMC4671 is only a motion controller. A 3-phase motor driver, such as the TMC6100 or TMC6200, will be needed as well to drive current to the motor.