The Nightstick Subcompact Weapon Light is not compatible with the Glock 19 Gen 3. This light was specifically designed for non-railed versions of the Glock 43, 43X, and 48. If you are looking for a light and laser combo, I would recommend the Streamlight TLR-8A Weapon Light and Laser which is compatible with the Gen 3 Glock 19 and can be found by using the link below:
https://www.opticsplanet.com/streamlight-tlr-8-a.html