Three is a good number. C needs to be one of them, because everyone writes their libraries in C, and you need to use them and be able to contribute your own libraries. Don't use C for applications, though; there is no performance gain (gluing C libraries together in C is as fast in $foo as it is in C for most values of $foo), and there is a maintainability and expressiveness loss. Libraries are simple enough and smal…
I recently learned that Lua is pretty fast, according to anecdote and shoot-out data. I did not like the syntax, when I last looked at it. But I might change my might. I mostly use Python for this class of things at the moment. Though I could see myself use Haskell more.
We use OCaml at work for your first and second category. (We do interface with C, too.)