function getFirstKeyInLevel(json, level) {
    var levelNow = 0;
    var key;
    var obj = json;
    do {
        key = Object.keys(obj)[0];
        levelNow++;
        obj = obj[key];
    } while (key && levelNow < level);
    return key;
}
var json = {
    'RuntimeSources': {
        'flask-webapp': {
            'eb-flask1.3': {
                's3url': ''
            }
        }
    },
    'DeploymentId': 4,
    'Serial': 4
};

console.log(getFirstKeyInLevel(json, 1)); // RuntimeSources
console.log(getFirstKeyInLevel(json, 2)); // flask-webapp
console.log(getFirstKeyInLevel(json, 3)); // eb-flask1.3
console.log(getFirstKeyInLevel(json, 4)); // s3url
console.log(getFirstKeyInLevel(json, 5)); // undefined