variables. global variables.
3 global variables called obj1, obj2 and obj3. in the init.sqs file set them all to false. when the first objective is completed, in whatever trigger senses it put obj1 = true. same with the second and third.
in your end trigger condition field, put
obj1 and obj2 and obj3
that simple.
hiding a marker. find out what type it is (warning, objective, target, etc) and remember. then in the init.sqs file set its type to empty, like this
"my_marker" setmarkertype "empty"
it will vanish, but it will still be there. when you want it to appear again, reset its type back to what it was.