hidden markers:
place your marker, make it type empty, and when you need it to be visible use:
"MarkerOne" setMarkerType "Arrow"
hidden objectives:
to make the 3rd objective hidden at the beginning, put this into your init.sqs
"3" objstatus "HIDDEN"
to make it show up, put the following onto a trigger or a script:
"3" objstatus "ACTIVE"