Under eye twitching around 3 week after Restylane filler to tear trough. Is this normal?

It is not normal for filler to cause twitching in the eye. You may want to consider consulting a optometrists and discuss your concerns. All the Best. 

I would not consider this "normal" but I have seen it happen. It usually occurs when a nerve was traumatized during the procedure. It will resolve on its own but it can take some time. You might want to re-visit your injector just to be assured that's what it is. "This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."