Filter Gridview via Multiple dropdownlist and Trigger the filter action after clicked a button

Hi,
   I'm currently working on a project where I would like to create a search engine page where in the page I have three dropdownlist filters and a button. At the beginning, a gridview will be populated with records from a database table. The user can filter down the gridview data accordingly to the dropdownlist filters selection. But all the filter action will only be triggered after clicked on a Filter button.
These are the dropdownlist filters's items and button:
Dropdownlist filters:
Employee (All, Gina, Lina, David) - the employee names will changed accordingly based on the logged in user which is supervisor.
Leave Type (All, Annual Leave, Medical Leave, Exam Leave)
Status (All, Pending, Pending for cancel, Approved, Rejected, Cancelled)
Button: Filter

For dropdownlist Employee and Leave Type, I have retreived the data for ...