DropDown Box and SQL

OK...Here goes.

I have three drop down boxes that I want to populate with data from my db.  This is no big deal,
But, I want to filter the results on each dropdown box.  For example, Row, Bay and Shelf (all in same table) are the columns I want
to display.  I created a dropdown box for each one.  They automatically display the contents of the entire db.  
When a user picks a specific Row, I want the Bay and Shelf information updated in their dropdown boxes.  Same thing if a user picks Bay.

Row      Bay      Shelf
1      A      1
1      A      2
1      B      1
2      C      3
2      C      3
2      C      3
3      D      4
3      D      4
3      D      2

Here is a example of the data.  When a user picks Row "1",  Bay A & B and shelves 1 & 2 should only be displayed.


Any Ideas?

Thanks.
sw