What node is the player and what object will be pushing the player? Its hard to say without more details.
Generally, when I have done this in 3D with KinematicBody nodes, it's just a matter of detecting the collision, the normal of the collision (use a raycast), and the velocity the object is moving at. Then to push the character, set the character's velocity to the direction of the normal multiplied by the velocity of the moving object. That's the simplest way to do it that I have found and I think it worked decently in most cases (though there were edge cases)
