The below sample code design based on your requirement.

pyhton code:-

class sample_order(osv.osv):
    _name = "sample.order"
    _columns = {
       'name'    : fields.char('Name',  size=64),
       'order_line': fields.one2many('sample.order', 'line_id', 'Order Lines'),
        }

class sample_order_line(osv.osv):        
    _name = "sample.order.line"
    _columns = {
       'name'    : fields.char('Name',  size=64),
       'order_subline': fields.one2many('sample.order.line', 'subline_id', 'Order Lines'),
       'line_id'       : fields.many2one('sample_order', 'Line'),
        }    
 
class sample_line_subline(osv.osv):
_name = "sample.line.subline"
    _columns = {
       'name'    : fields.char('Name',  size=64),
       'subline_id'       : fields.many2one('sample.order.line', 'Line'),
               }    
             

xml file:-

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
    <record id="view_sample_order_form" model="ir.ui.view">
      <field name="name">sample.order.form</field>
      <field name="model">sample.order</field>
      <field name="type">form</field>
      <field name="arch" type="xml">
      <form string="Sample Order">
        <field name="name"/>
        <notebook colspan="4">
          <page string="Order Line">
           <field name="order_line" nolabel="1" colspan="4">
            <form string="Order">
               <field name="name"/>
                <notebook colspan="4">
                   <page string="Sub Line">
                     <field name="order_subline" nolabel="1" colspan="4">
                      <form string="Sub Line">
                         <field name="name"/>
                      </form>
                      <tree string="Sub Line">
                         <field name="name"/>
                      </tree>
                      </field>
                    </page>
                </notebook>
            </form>
            </field>
            </page>    
        </notebook>
        </form>
        </field>
    </record>
    </data>
</openerp>