I am trying this..

Imports System.Data.OleDb

Public Class Form1
    Dim conn As OleDbConnection
    Dim cmd As OleDbCommand

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        conn = New OleDbConnection("Provider=Microsoft.Jet.Oledb.4.0; Data Source=c:\nikhil.mdb")
        Dim row As DataRowView=ComboBox1.SelectedValue

        Dim row1 As DataRowView = ComboBox2.SelectedValue
        Dim row2 As DataRowView = ComboBox3.SelectedValue

        cmd = New OleDbCommand("insert into table1 values( " & row.Row("id").ToString & ",' " & row1.Row("name").ToString & " ', " & row2.Row("salary").ToString & ")", conn)


    End Sub

I get an error: object reference not set to an instance of an object

I guess one of the combobox is not selected and variables "row", "row1" or "row2" has nothing (Null).

So use IsNothing method to test an object is missing or not.

If IsNothing(row)  Then
   //Select 1st combobox value
 End If
 If IsNothing(row1)  Then
   //Select 2nd combobox value
 End If