Hi shaks,

I have created a sample which full fill your requirement

C#

protected void OnItemChange(object sender, EventArgs e)
{
    int rowIndex = 0;
    TextBox box1 = new TextBox();
    TextBox box2 = new TextBox();
    TextBox box3 = new TextBox();
    for (int i = 0; i < Gridview1.Rows.Count; i++)
    {
        box1 = (TextBox)Gridview1.Rows[rowIndex].Cells[1].FindControl("TextBox1");
        box2 = (TextBox)Gridview1.Rows[rowIndex].Cells[2].FindControl("TextBox2");
        box3 = (TextBox)Gridview1.Rows[rowIndex].Cells[3].FindControl("TextBox3");
        rowIndex++;
    }
    if (!string.IsNullOrEmpty(box1.Text.Trim()) && !string.IsNullOrEmpty(box2.Text.Trim()) && !string.IsNullOrEmpty(box3.Text.Trim()))
    {
        AddNewRowToGrid();
    }
    else
    {
        Response.Write("<script type=\"text/javascript\">alert('Please Fill the Details');</script>");
    }
}

HTML

<div>
    <asp:GridView ID="Gridview1" runat="server" ShowFooter="true" AutoGenerateColumns="false">
        <Columns>
            <asp:BoundField DataField="RowNumber" HeaderText="Row Number" />
            <asp:TemplateField HeaderText="Header 1">
                <ItemTemplate>
                    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Header 2">
                <ItemTemplate>
                    <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
                </ItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Header 3">
                <ItemTemplate>
                    <asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
    <asp:DropDownList ID="ddlItem" AutoPostBack="true" runat="server" OnSelectedIndexChanged="OnItemChange">
        <asp:ListItem Text="Item1" />
        <asp:ListItem Text="Item2" />
        <asp:ListItem Text="Item3" />
    </asp:DropDownList>
</div>

and refer below article

Add new Row to GridView on Button Click in ASP.Net

ScreenShot

Hope this works for you