fk problems with 0..n relations

I there, i'm having a problem. i have a table(1) that is connected with two multiple table (2 and 3). the problem is that the table 1 might have one of the tables (2 or 3, or both). how can i make this relationship in sql. Resuming, table 1 as a 0..n relation with table 2 and 3. thanks in advance.