Access the x, y, z components using [0], [1], [2] respectively.

	var p : Vector3;
	p[1] = 5; // the same as p.y = 5

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public Vector3 p;
    void Example() {
        p[1] = 5;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public p as Vector3

	def Example() as void:
		p[1] = 5