Depending Table.

You can have two dependent dropdowns at most. As workaround, you can implement a jquery solution like this: http://stackoverflow.com/questions/5910281/jquery-dependent-drop-down-boxes-populate-how

the problem is i have 300+ sport event.

so, i cant use the solution that u have suggest.

do you have another suggestion to solve my problem?

In fact you can have unlimited number of chained dependent dropdown boxes. Check 'Creating multiple chained dropdown boxes' tutorial at http://xlinesoft.com/phprunner/php-database.htm

The problem is in dependent list only the field in the main table exist. The field that contain the dependent field doesn't exist in dependent list.
The relationship between the table is like below:
main_table > athletesports_table > sport_table,event_table,position_table
in the main page which contain main_table, there is a tab for athletesports_table which contain information about athlete's sport. and in athletesports tab, the 3 table is related and i want the 3 table depend each other. why in the dependent list cannot display information from 3 last tables as above?

I recommend to post what you have already to Demo Account and contact support directly so we can see what you have built already and what's missing. It's not easy to decipher based on your description.