[Vuejs]-Getting the value of the second selectbox to fetch result

Make a new object that has properties for each select box to mutate:

selections: {
    iller: null,
    ilceler: null,
    mahalleler: null
}

Update all your v-model bindings:

v-model="selections.iller"
v-model="selections.ilceler"
v-model="selections.mhalleler"

Now you can get them correctly.