3 entries going into 1 field

Hi, I have a registration form, but I would like to add a Date of Birth section. Now, because of the different formats of writing it I want to have 3 seperate drop down boxes, Date, Month and Year. However, I then want those 3 dropdowns to go into one field in a mySQL database, in DD/MM/YYYY format. How can I acheive this, in simple language as I'm a newbie to this stuff ^_~ Thanks! Vicky