Change datagrid cell color of every third cell

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C#

Hi i need to change every third cell color into red its a 4 by 4 datagrid i struggle with what to use as my row and column index? this is what ive got

private void btnred_Click(object sender, EventArgs e)
{
    int teller;

    for (teller = 1; teller <= 16; teller++)
    {
        DataGridViewCellStyle CellStyle = new DataGridViewCellStyle();
        CellStyle.BackColor = Color.Red;
        dgvArray.Rows[(this parts i dont know)].Cells[].Style = CellStyle; 
        teller += 1;
    }
}