RandomPosition3D.cs

using UnityEngine;

public class RandomPosition3D : MonoBehaviour
{
    [SerializeField]
    private Color GizmosColor = new Color(0.5f, 0.5f, 0.5f, 0.2f);

    private float xRange = 10f, yRange = 10f, zRange = 10f;
    private float Range(float scale, float range) => scale + Random.Range(-range, range);
    
    private void OnDrawGizmos()
    {
        Gizmos.color = GizmosColor;
        Gizmos.DrawCube(transform.position, transform.localScale);
    }

    private void Awake()
    {
        Initialize();
    }

    //Gets stats from localscale of your spawn area
    private void Initialize()
    {
        xRange = transform.localScale.x / 2f;
        yRange = transform.localScale.y / 2f;
        zRange = transform.localScale.z / 2f;
    }

    ///<summary>
    ///Will choose a random position inside area choosing a random X, Z and with fixed Y value.
    ///</summary>
    public Vector3 SpawnPositionCustomHeight(float customY = 0)
    {
        var position = transform.position;
        return new Vector3(Range(position.x, xRange), customY, Range(position.z, zRange));
    }

    ///<summary>
    ///Will choose a random position in X, Y Z.
    ///</summary>
    public Vector3 SpawnPosition()
    {
        var position = transform.position;
        return new Vector3(Range(position.x, xRange), Range(position.y, yRange), Range(position.z, zRange));;
    }
}