Node.js Nested Callbacks, multiple requests
Use Step.js or async.js to make this look cleaner.  Step.js is simpler, but async.js gives you more flexibility.  

Also, your function single needs to have a callback, and cannot return values since the functions it's invoking are not returning values.

Additionally, generally the data retrieval methods should be separate from the rendering methods.

With Step.js, this could look like:

 var Parse = require('parse-api').Parse,
    Step = require('step');
    var parse = new Parse();
exports.getAndRenderData = function(res, params1, params2, params3){
   getData(params1, params2, params3, function(err, data){
      if(err)
        throw err:
      else {
        res.render('viewname', data);
      }
   });
}

var getData = function(params1, params2, params3, callack) {
    var data = {}; 
    Step()(
        function getClass1(){
            parse.find('Class_1',params1, this);
        },
        function getClass2(err, class1data){
           if (err) throw err;
           data.class1 = class1data;
           parse.find('Class_2',params2, this);
        },
        function getClass3(err, class2data){
           if (err) throw err;
           data.class2 = class2data;
           parse.find('Class_3',params3, this);
        },
        function callbackWithAll(err, class3data){
           if (err) callback(err); 
           else {
             data.class3 = class3data;
             callback(null, data);
           }
        }
    );
}