3 drop down updation on change of 1 drop down

I am a newbie and I am wanna use 3 drop down

1 Country drop down

2 State drop down

3 City drop down

so onchange of country drop box the other 2 drop down is updated by database

Please help.....with any reference or sample code

Thank you in advance..

See, for example, this page Changing Select element content on the fly

Thank you