from smlr
First, create your AAdmin, ABAdmin and BAdmin Classes like you would, if those Classes wouldn't share any relation. After this, add to your sonata.admin.A service a Child sonata.admin.AB (Admin.yml or Admin.xml).
Than you can use the configureSideMenu() method in AAdmin to create a sidemenu and generate a link to ABAdmin.
It's important to add $parentAssociationMapping to your ABAdmin Class and adding some if-statements to improve the usability, if list and form is in a A-context.
If you know the mechanism, it's quite easy. Hope my example works for you..