how to set unique primary key for all table in same database

You may add a new table to your schema called ID_Table that will have only one numeric column called current_id with default value of 0 ,when adding a new row to any other table of the schema you have to call a select on the ID_Table returning ID_Table.current_id + 1 as new id value.Then updating ID_Table must be done

Update ID_Tableset ID_Table.current_id = ID_Table.current_id + 1 

the GetNewId function could be implemented bylocking the ID_Table
Updating ID_Table
returning NewID

something like this (I have used Oracle syntax)

create table ID_Table(   current_id number); Insert into ID_Table values(0);CREATE OR REPLACE Function GetNewId RETURN number is  new_id    ID_Table.current_id%type;  row_count number;begin  select nvl(ID_Table.current_id, 0) + 1    INTO new_id    FROM ID_Table     for update;  update ID_Table set ID_Table.Current_Id = new_id;  commit;  RETURN new_id;end GetNewId;

You can get maximum ID from all three tables then add it in your insert query. But you have to remove the auto_increment attribute.

INSERT INTO TableASELECT MAX(ID)+1, 'jjj'  FROM      (SELECT MAX(ID) AS ID FROM TableA       UNION       SELECT MAX(ID) AS ID FROM TableB       UNION       SELECT MAX(ID) AS ID FROM TableC      ) A;

See this SQLFiddle

Use the same sequence as the id generator for each inserted row, regardless of the table. Assuming you're using a DB that allows a sequence to be named as the id generator for the field.

This looks like it will do what you want in MySQL: http://devzone.zend.com/1786/mysql-sequence-generator/