Appliction master page having 3 drop down control in want to take this value and fill my gridview but gridview are in other page

there are serveral ways you can do this
first
use ((dropdownlist)master.findcontrol["name of your control"]).selectedvalue

use this for all your three dropdownlist

or
you can store selectedvalue of dropdownlist in session variable
like this

session["firstdropdownlist"] = dropdownlist1.selectedvalue
like this for other dropdownlists

and then on the page it is required

if(session["firstdropdown"] != null)
{
string value = convert.tostring(session["firstdropdown"])
}