You need to use the 'variables' field in the URLRequest object:
var request:URLRequest = new URLRequest(url);
request.method = URLRequestMethod.POST;
var variables:URLVariables = new URLVariables();
variables.param1 = ...
variables.param2 = ...
variables.param3 = ...
request.data = variables;
var urlLoader:URLLoader = new URLLoader();
urlLoader.dataFormat = URLLoaderDataFormat.VARIABLES;
urlLoader.addEventListener(Event.COMPLETE, urlLoader_complete);
urlLoader.addEventListener(IOErrorEvent.IO_ERROR, urlLoader_error);
urlLoader.load(request);
Also, it's a good idea to define handlers for errors (as shown for IO_ERROR above but also HTTP_STATUS and SECURITY_ERROR)