Sql Transactions Tiers Applications

Hi All.. I have an 3 tier web applications and I want to use sql transactions but not in each class.

Example :

I have 3 clases who need to update each table in one transaction how I do use sql transactions. I try to do in this way but not work:

sub cmdupdate_click
   dim a as new class1
   dim b as new class2
   dim c as new class3
   dim cn as new sqlconnection......
   cn.open
   .....transactions.... scripts
   a.cn =cn ...I pass the same connection via property to each class
   a.update
   b.cn=cn....
   b.update
   c.cn=cn.....
   c.update

   Transaction...scripts to commit
   cn.close
end sub


maybe that logic is incorrect.  I dont now is my explain is clear.