chrome.storage.sync.set({
      x: document.getElementById('a').value,
      y: document.getElementById('b').value,
      z: document.getElementById('c').value
    }, function() {...});

var baseA = "";
var baseB = "";
var baseC = "";

chrome.storage.sync.get({
    x: "",
    y: "",
    z: ""
}, function(items) {
    setValues(items.x, items.y, items.z);
});

function setValues(v1,v2,v3) {
    baseA = v1;
    baseB = v2;
    baseC = v3;
    console.log('baseA | ' + baseA);
    console.log('baseB | ' + baseB);
    console.log('baseC | ' + baseC);
}

function doSomething() {
    if (baseA !="") {
        console.log('true ' + baseA);
    }
    else {console.log('false')}
}

doSomething();

x = "aaa";
y = "bbb";
z = "ccc";

false
baseA | aaa
baseB | bbb
baseC | ccc