try like this
dataGrivView1[0,0].Value = txtbox1.Text;
or
assign the textbox values to datarow like
DataRow rd;
rd[0]=txtbox1.text;
and declare a data table
datatable dt=new datatable;
dt.rows .add(rd);
dataGridView.Datasource=dt;
or
DataGridViewRow row = new DataGridViewRow();
row.CreateCells(this.dataGridView1, textBox1.Text, textBox2.Text, textBox3.Text);
this.dataGridView1.Rows.Add(row);