It seems you are looking for such layout:

To do so, you don't need to create any custom control. Just use common features of ToolStrip
. You need to set the LayoutStyle
property to a suitable value.
Example
private void Form1_Load(object sender, EventArgs e)
{
var dropdown = new ToolStripDropDown();
//Define style
dropdown.LayoutStyle = ToolStripLayoutStyle.Table;
var settings = (dropdown.LayoutSettings as TableLayoutSettings);
settings.ColumnCount = 3;
//First Item
var item1 = new ToolStripMenuItem("Some Sub Menu");
dropdown.Items.Add(item1);
settings.SetColumnSpan(item1, 3); //Set column span to fill the row
//First Combo
var combo1 = new ToolStripComboBox("combo1");
combo1.Items.AddRange(new string[] { "Item1", "Item2", "Item3" });
dropdown.Items.Add(combo1);
//Separator
dropdown.Items.Add("-");
//Second Combo
var combo2 = new ToolStripComboBox("combo2");
combo2.Items.AddRange(new string[] { "Item1", "Item2", "Item3" });
dropdown.Items.Add(combo2);
//Last item
var item2 = new ToolStripMenuItem("Some Othe Sub Menu");
dropdown.Items.Add(item2);
settings.SetColumnSpan(item2, 3); //Set column span to fill the row
toolStripDropDownButton1.DropDown = dropdown;
}