Try Like This
Protected Sub GridView1_RowCommand(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.GridViewCommandEventArgs) Handles GridView1.RowCommand
If e.CommandName = "select" Then
Dim i As Integer
i = e.CommandArgument()
Dim row As GridViewRow
row = GridView1.Rows(i)
TextBox1.Text= row.Cells(1).Text
TextBox2.Text = row.Cells(2).Text
TextBox3.Text = row.Cells(3).Text
End If
End Sub