Входная строка имела неверный формат

if (line_t.LastIndexOf(',') > 0)
                         {
                            masiv_koordinat[0] =  Convert.ToDouble(line_t.Substring(0, line_t.Length -  line_t.IndexOf(",")-3));
                             masiv_koordinat[1] =  Convert.ToDouble(line_t.Substring(line_t.IndexOf(",") + 1,  line_t.LastIndexOf(",") - line_t.IndexOf(",")+1));
                             masiv_koordinat[2] =  Convert.ToDouble(line_t.Substring(line_t.LastIndexOf(",")+1,line_t.Length-line_t.LastIndexOf(",")));
                         }

line_t.Substring(0, line_t.Length -  line_t.IndexOf(",")-3)    "-15.93088247"    string

CultureInfo c = new CultureInfo("ru"); // or another culture
c.NumberFormat.NumberDecimalSeparator = ".";
var result = Double.Parse("-15.93088247", c);

using System.Globalization;
 
var coord = "-56.45645,69.565654,0.56898989454";
var vector3 = coord.Split(',');
var vector_x = double.Parse(vector3[0], CultureInfo.InvariantCulture);
var vector_y = double.Parse(vector3[1], CultureInfo.InvariantCulture);
var vector_z = double.Parse(vector3[2], CultureInfo.InvariantCulture);

public struct Vector3
{
    public double X;
    public double Y;
    public double Z;
 
    public Vector3(string str, char split)
    {
        var vector3 = str.Split(split);
        X = double.Parse(vector3[0], CultureInfo.InvariantCulture);
        Y = double.Parse(vector3[1], CultureInfo.InvariantCulture);
        Z = double.Parse(vector3[2], CultureInfo.InvariantCulture);
    }
 
    public override string ToString()
    {
        return string.Format("X={0}, Y={1}, Z={2}", X, Y, Z);
    }
}