I have this table product and it is linked with three tables. So I tried to assign foreign key product_no to three different tables which don't work. I successfully assigned foreign key to one of the table among three.

"don't work" is not a valid Oracle error message.

show us the CREATE TABLE for the tables involved
show us the statement you tried to execute
show us the exact error message you get.

and make sure your SQL code is formatted using [code] tags to make it readable.

For details on the tags see the help:
http://www.dbforums.com/misc.php?do=bbcode#code

Product table:
CREATE TABLE product (
model_no NUMBER NOT NULL,
manufacturer_no NUMBER,
product_desc VARCHAR2(12),
product_type VARCHAR(10),
unit_price NUMBER,
CONSTRAINT product_pk PRIMARY KEY(model_no)
);
product table is created successfully.
CREATE TABLE computer (
model_no NUMBER,
speed NUMBER,
memory NUMBER,
unit_price NUMBER,
CONSTRAINT fk_product
FOREIGN KEY (model_no)
REFERENCES product(model_no)
);
the foreign key is added successfully in one of the three tables::
In other tables when I try to add model_no foreign key using alter syntax
ALTER TABLE accessories
add CONSTRAINT fk_product
FOREIGN KEY(model_no)
REFERENCES product(model_no);

I get this error:
ORA-02264: name already used by an existing constraint

Well the error message says it all....

(And please use [code] tags the next time)

that's funny, we get the same error message in sql server and mysql, too

my car doesn't work.
tell m how to make my car go.

That means, I cannot add foreign key in more than one table using the same primary key reference?
I want this answer.
Can I or Can't I?

yes, you can

how?
I tried to add but got the error as you have already seen.
How can I add?

What about, well, following the error message? It is very descriptive, is not it?
http://ora-02264.ora-code.com/
Change your naming convention to make constraint names (those identifiers in bold) different.