これでうまくいくはずです。異なる列を持つために複数のテーブルは必要ありません。Wordでは、最初の行に2列、次の3行に5列、最後の3行に1列しかない単一のテーブルを作成できます。(5列で必要な行数を言わなかったので、3行にしました。)
//Be sure to add this reference:
//Project>Add Reference>.NET tab>Microsoft.Office.Interop.Word
//open Word App
Microsoft.Office.Interop.Word.Application msWord = new Microsoft.Office.Interop.Word.Application();
//make it visible or it'll stay running in the background
msWord.Visible = true;
//open a new document based on the Word template.
//You shouldn't open the template directly using msWord.Documents.Open(path) unless you want to edit the template itself.
Microsoft.Office.Interop.Word.Document wordDoc = msWord.Documents.Add(@"c:\MyTemplate.dotx");
//find the bookmark
string bookmarkName = "BookmarkToFind";
if (wordDoc.Bookmarks.Exists(bookmarkName))
{
Microsoft.Office.Interop.Word.Bookmark bk = wordDoc.Bookmarks[bookmarkName];
//set the document's range to immediately after the bookmark.
//If you want to add the table *into* the bookmark, it needs to be done differently.
//This page has a good explanation of the differences between adding to the bookmark's range vs adding after the bookmark's range.
//http://gregmaxey.mvps.org/word_tip_pages/insert_text_at_or_in_bookmark.html
//It's a little more hassle because you have to re-add the bookmark after inserting into it,
//so inserting after the bookmark is usually fine less you're going to be inserting text programmatically at the same bookmark a second time.
Microsoft.Office.Interop.Word.Range rng = wordDoc.Range(bk.Range.End, bk.Range.End);
//create a table with 8 rows and 5 columns into the range.
Microsoft.Office.Interop.Word.Table tbl = wordDoc.Tables.Add(rng, 8, 5);
//set the table's borders.
tbl.Borders.InsideLineStyle = Microsoft.Office.Interop.Word.WdLineStyle.wdLineStyleSingle;
tbl.Borders.OutsideLineStyle = Microsoft.Office.Interop.Word.WdLineStyle.wdLineStyleSingle;
//merge the cells in the first row down to 2 columns (Word's cells start at 1, not at 0).
tbl.Cell(1, 1).Merge(tbl.Cell(1, 3));
//distribute the columns evenly
tbl.Rows[1].Select();
msWord.Selection.Cells.DistributeWidth();
//rows 2-5 already have 5 columns so don't touch them.
//merge rows 6-8 into single-columns rows.
for (int x = 6; x < 9; x++)
{
tbl.Cell(x,1).Merge(tbl.Cell(x,5));
}
//put the cursor in the table's first cell.
rng=wordDoc.Range(tbl.Cell(1,1).Range.Start, tbl.Cell(1,1).Range.Start);
rng.Select();