try one of these ...

SQL> create table my_table (col1 char(1),col2 number,col3 date);

Table created.

SQL> insert into my_table (col1) values ('A');

1 row created.

SQL> insert into my_table (col1,col2,col3) values ('A',null,null);

1 row created.

SQL> insert into my_table (col1,col2,col3) values ('A',to_number(null),
2 to_date(null));

1 row created.

SQL> select * from my_table;

C COL2 COL3
- ---------- ---------
A
A
A

SQL>