Bulid a application for mac , windows and linux

I have a website related to cloud storage. I need application similar to what dropbox use. Not exactly i dont like copying. Let me tell how it works. Files are stored on some server that is done by my website. It has api too. What i need is to create a software that can access that api and access , download ,upload and show the files in it. I think you get the point, All the files details, uploading and downloading is done or sent by api ,so dont worry about that . Login is mandatory from client side on software , thought its obvious i just want to let you know. Also it will be better if software is nice looking and awesome interface and fully secure too. If you can create in c++. it will be best as i know that and can fix something i want in software myself. Thanks