Use a coroutine that moves from one point to another, inside an infinite loop:

var pointB : Vector3;

function Start () {
    var pointA = transform.position;
    while (true) {
        yield MoveObject(transform, pointA, pointB, 3.0);
        yield MoveObject(transform, pointB, pointA, 3.0);
    }
}

function MoveObject (thisTransform : Transform, startPos : Vector3, endPos : Vector3, time : float) {
    var i = 0.0;
    var rate = 1.0/time;
    while (i < 1.0) {
        i += Time.deltaTime * rate;
        thisTransform.position = Vector3.Lerp(startPos, endPos, i);
        yield; 
    }
}

Similar suggestion to Eric5h5's here, but simplified somewhat, making use of Mathf.PingPong, speed is adjustable in the inspector, and you can use an empty gameobject reference to define the location of PointB.

var pointB : Transform;
private var pointA : Vector3;
var speed = 1.0;

function Start () {
    pointA = transform.position;
    while (true) {
        var i = Mathf.PingPong(Time.time * speed, 1);
        transform.position = Vector3.Lerp(pointA, pointB.position, i);
        yield;
    }
}

(untested, don't have unity in front of me right now)

For Those who need it written in c# i believe this should work.

public Vector3 pointB;

IEnumerator Start () {
    Vector3 pointA = transform.position;
    while (true) {
        yield MoveObject(transform, pointA, pointB, 3.0);
        yield MoveObject(transform, pointB, pointA, 3.0);
    }
}

IEnumerator MoveObject (Transform thisTransform, Vector3 startPos, Vector3 endPos, float time) {
    float i = 0.0f;
    float rate = 1.0f / time;
    while (i < 1.0f) {
        i += Time.deltaTime * rate;
        thisTransform.position = Vector3.Lerp(startPos, endPos, i);
        yield; 
    }
}

Can someone please convert this to C#? I'm having trouble trying to do it myself.

Show example of How can I call this fucntion? function MoveObject (thisTransform : Transform, startPos : Vector3, endPos : Vector3, time : float) (Eric5h5's func.)