A single language for all platforms does exist.
“If you are developing something for customers to use then in an ideal world you would support all three platforms, but since there is no single language that works on all three you find yourself having to employ either multi-skilled developers (a scarce resource) or more bodies, each of which has more focused skills.”
Not exactly accurate, AppMethod (http://www.appmethod.com) provides a mobile development platform where you design & write in a single language and then compile to multiple platforms.
No affiliation with the product or company but have played with it over the trial period and successfully compiled for iOS and Android. You need to know a different language from what the native compilers use (Object Pascal or C++) and I'm sure cutting edge features lag behind but if you're making an app for all 3 platforms you could do a lot worse.