Because of the asynchronous nature of Node.js if you make three HTTP requests with three different nonce values like the following
GET http://example.com/?nonce=1
GET http://example.com/?nonce=2
GET http://example.com/?nonce=3
All three request will happen concurrently. Which ever request gets a response back from the server will be the first to complete (i.e its callback will run).
You could incorporate the functions of the async module like async.series or async.map to ensure the requests return in order.