Alternative methods of making character move with 3d moving platform?

A simple solution would be to set the player velocity to match the moving platform velocity when the player lands.

 void OnCollisionEnter(Collision collision){
      if( collision.gameObject.tag == "Platform" ){
            gameObject.GetComponent<Rigidbody>().velocity = collision.gameObject.GetComponent<Rigidbody>().velocity;
      }

Edit: To get this working in my test, I had a cube acting as the platform and a capsule as the player, both with rigidbodies attached. The only settings I changed on the rigidbodies were to set the platform rigidbody to kinematic and to constrain the x and z rotation of the capsule so it wouldn't roll over.

On the capsule, I added these two scripts:

 using UnityEngine;
 using System.Collections;
 
 public class MovePlayer : MonoBehaviour {
     Rigidbody rbody;
     
     void Start () {
         rbody = gameObject.GetComponent<Rigidbody>();
     }
     
     void FixedUpdate () {
         float xAxis = Input.GetAxis("Horizontal");
         rbody.AddForce(Vector3.right * 20 * xAxis);
     }
 }

and

 using UnityEngine;
 using System.Collections;
 
 public class PlayerCollision : MonoBehaviour
 {
     void OnCollisionEnter(Collision collision)
     {
         print("CollisionEnter");
         gameObject.GetComponent<Rigidbody>().velocity = collision.gameObject.GetComponent<Rigidbody>().velocity;
     }
 }

On the platform, I used this one script:

 using UnityEngine;
 using System.Collections;
 
 public class MovePlatform : MonoBehaviour {
     Rigidbody rbody;
     public Vector3 targetPos = new Vector3(5, 0, 0);
     float elapsedTime = 0f;
     
     // Use this for initialization
     void Awake() {
         rbody = gameObject.GetComponent<Rigidbody>();
     }
     
     // Update is called once per frame
     void FixedUpdate () {
         elapsedTime += Time.deltaTime;
         rbody.MovePosition(targetPos * elapsedTime );
     }
 }
 

To simulate dropping onto the platform, I started the capsule a short distance above the platform. The result was a capsule that didn't slide at all when it landed on the platform and was still able to move with my input.