Open Window Form.Add on DataGridview and three Textboxes on windows Form.
In the above example I am created on emp table and having 3 columns as id number,empname(varchar (20),salary number(20).
In form load Retrive the data a emp Table and fill in the Datagridview.When you click or Select a Row on DataGridview the particular selected row values will display in Textboxes by using below code.

i = dataGridView1.SelectedCells[0].RowIndex;
textBox1.Text = dataGridView1.Rows[i].Cells[0].Value.ToString();
textBox2.Text = dataGridView1.Rows[i].Cells[1].Value.ToString();
textBox3.Text = dataGridView1.Rows[i].Cells[2].Value.ToString();

before using this code you have to double click your DataGridview and write the  code in
between the 

Private void dataGridView1_CellContentClick_(objectsender,DataGridViewCellEventArgs e)
{

}
 

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;
namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        SqlDataAdapter da;
        DataSet ds;
        int i;
        SqlConnection conn;
        private void Form1_Load(object sender, EventArgs e)
        {           
            conn = new SqlConnection("connetion tring");
            conn.Open();
            da= new SqlDataAdapter("select * from emp", conn);
            SqlCommandBuilder builder = new SqlCommandBuilder(da);
            ds = new DataSet();
            da.Fill(ds, "emp");
            dataGridView1.DataSource = ds.Tables["emp"];
        }

Private void dataGridView1_CellContentClick_1(objectsender,DataGridViewCellEventArgs e)
    {
i = dataGridView1.SelectedCells[0].RowIndex;
textBox1.Text = dataGridView1.Rows[i].Cells[0].Value.ToString();
textBox2.Text = dataGridView1.Rows[i].Cells[1].Value.ToString();
textBox3.Text = dataGridView1.Rows[i].Cells[2].Value.ToString();
  }
}