I want create a app for the 3 princiapl OS: Android, WindowsPhone and iOS.
But all in one app. I think use Xamarin (whith Visual Studio C#).

Can i do it using Backendless?

Hi João,
We have .NET SDK, so if you can use it in your Xamarin project - then yes, you can do it with Backendless. But note that our .NET SDK is not actively developed, so I would suggest you either using a dedicated SDK for each platform or try luck with some JS framework and our JS SDK.