List<bitmap> bitmaps = new List<bitmap>();
Size size = new Size();

for (int i = 1; i < 4; ++i) 
{ 
	// lookup the datagridview 
	DataGridView dataGridView = (DataGridView)this.Controls["dataGridView" + i]; 
	// maximize the datagridview size (choosing between current and preferred) 
	Size oldSize = dataGridView.Size; 
	Size newSize = dataGridView.PreferredSize; 
	if (dataGridView.Size.Width > newSize.Width)
	{
		newSize.Width = dataGridView.Size.Width;
	}

	if (dataGridView.Size.Height > newSize.Height)
	{
		newSize.Height = dataGridView.Size.Height;
	}

	dataGridView.Size = newSize;

	// draw the datagridview into a bitmap
	Bitmap bitmap = new Bitmap(dataGridView.Width, dataGridView.Height);
	dataGridView.DrawToBitmap(bitmap, new Rectangle(0, 0, dataGridView.Width, dataGridView.Height));

	// restore the datagridview to it's original size
	dataGridView.Size = oldSize;

	bitmaps.Add(bitmap);

	// update total bitmap size
	if (newSize.Width > size.Width)
	{
		size.Width = newSize.Width;
	}
	size.Height += newSize.Height;
}

// copy all the bitmaps into one large bitmap
Bitmap bitmapComplete = new Bitmap(size.Width, size.Height);
Graphics g = Graphics.FromImage(bitmapComplete);

int height = 0;
for (int i = 1; i < 4; ++i) 
{ 
	g.DrawImageUnscaled(bitmaps[i - 1], 0, height); 
	height += bitmaps[i - 1].Height; 
} 

// bitmapComplete is ready for use 
// eg: pictureBox1.Image = bitmapComplete 
// eg: bitmapComplete.Save(@"C:\screenshot.jpg", ImageFormat.Jpeg);