Try setting a property in each pom to find the main project directory.
In the parent:
<properties>
<main.basedir>${project.basedir}</main.basedir>
</properties>
In the children:
<properties>
<main.basedir>${project.parent.basedir}</main.basedir>
</properties>
In the grandchildren:
<properties>
<main.basedir>${project.parent.parent.basedir}</main.basedir>
</properties>