The language isn't the issue, the frameworks and libraries are. You're going to have to spend your valuable expert time sorting those out too. That's the bulk of it.
If your truly an expert at c# then learning objective-c isn't much of a challenge. At least learning the amount you need to build apps.
The advantage of using a single language is that, for each single app in all the three store, you have to change a single codebase and not to make 3 different changes in 3 different codebases just because each of them require its own language (and then its own specific libraries).
So you have surely to spend the time to sort the libraries, but it is a one-time problem. After it, you can just use them without problem in all your projects and you really can focus on your idea
Put all the code in a good ui/codebehind separation framework like mvvm, and you are happier than ever.
See my comment above: I am under no delusions that cross-platform frameworks are truly cross-platform on the UI level. But, for our app, the UI layer is roughly 50% of the app. What about the core of the business logic? The code in a cross-platform framework is truly cross-platform. How do I approach that doing full native development for Android and iOS? Is there a way to create a cross-platform library?
And I can tell you, from my own experiences, your backend isn't going to be as cross platform as you dream of it being because everything from networking on down is done differently on Android and iOS. Which isn't to say that you can't front-end all of that with a shim in C# or C++ or whatevs, but on mobile, the bulk of the hard stuff is UI anyways and there is no cross-platform panacea as of this writing.
If your truly an expert at c# then learning objective-c isn't much of a challenge. At least learning the amount you need to build apps.