using System;
using System.Text;
using System.Linq;
using System.Collections.Generic;
using System.Reflection;
        
public class Program
{
public static void Main(string[] args)
{
    string insertString = @"INSERT INTO tb1(field1,field2,field3) VALUES (1,2,3);
                                            INSERT INTO tb1(field1,field2,field3) VALUES (4,5,6);
                                            INSERT INTO tb1(field1,field2,field3) VALUES (7,8,9);";
    IEnumerable<string> inserts = insertString.Split(new[] { Environment.NewLine }, StringSplitOptions.None);
    string firstInsert = inserts.First();
    int tableIndex = firstInsert.IndexOf("INSERT INTO ") + "INSERT INTO ".Length;
    string table = firstInsert.Substring(
        tableIndex, firstInsert.IndexOf("(", tableIndex) - tableIndex);
    var regex = new System.Text.RegularExpressions.Regex(@"\(([^)]+)\)", System.Text.RegularExpressions.RegexOptions.Compiled);
    string columns = regex.Matches(firstInsert)[0].Value;

    IEnumerable<string> values = inserts.Select(sql => regex.Matches(sql)[1].Value);

    string insertAll = string.Format("INSERT INTO {0}{1} VALUES {2};"
        , table
        , columns
        , string.Join(",", values.ToArray()));
        
        Console.Write("Result: " + insertAll);
}
}