Howto retrieve all the grandchildren of all the children

I have three tables. Table 1 contains the root of a tree. Table 2 contains
the children of table 1 (column acting as foreign key). Table 3 contains the
children of table 2 (column acting as foreign key). Basically it contains a
three level n-ary tree.

Is it possible to retrieve the grandchildren if I have the primary key of a
record in table 1, assuming that each table has a primary key and an
appropriate foreign key, and if so, what should be the SQL syntax to do so?

TIA

Joost Kraaijeveld