Limiting Distance

Good question. Your choice: Trigonometry or Transformation.

Trigonometry: What you need to do is detect the distance between your center object and the object that cannot be 3 units away.

If it is further away than it should be, calculate its positions angle in relative toe the center object, then use trigonometry to calculate a position at that angle exactly 3 units away!

Transformation: Use translation to move it back. How?

Subtract the position of the restricted object from the center object. Then get that vector's magnitude.

After that, multiply that magnitude by the distance between the two objects minus 3.

Finally, add the returned vector to transform.position :)

Code for transformation:

JS:

//Restricts the position of this object to be within a distance of "distance" of "center"
public var distance : float = 3;
public var center : Transform;
function Update ()
{
    var dst = Vector3.Distance(center.position, transform.position);
    if (dst > distance)
    {
        var vect : Vector3 = center.position  - transform.position;
        vect = vect.normalized;
        vect *= (dst-distance);
        transform.position += vect;
    }
}

C#:

using UnityEngine; using System.Collections;

 //Restricts the position of this object to be within a distance of "distance" of "center"
 public float distance = 3;
 public Transform center;
 private void Update ()
 {
     float dst = Vector3.Distance(center.position, transform.position);
     if (dst > distance)
     {
         Vector3 vect = center.position  - transform.position;
         vect = vect.normalized;
         vect *= (dst-distance);
         transform.position += vect;
     }
 }

Sorry I can't provide an example of the trigonometry version. Too complex in my opinion, and also really only works well in 2D.

If you are using Physics you can attach a SpringJoint to the objects and set Max Distance for the Spring Joint to 3 units. That will basically restrict an object from going beyond a 3 unit sphere. If you don't use gravity and set the Spring property to 0 it will just float around until it reaches it's limit. You can play around with the settings a bit to get it to work with your project.

This will invariably be faster than most any script you can write in JS or C# and will likely save you some time writing the math for it.