#[Swift 3](https://www.swift.org), 57 bytes

<!-- language-all: lang-swift -->

    func f(a:[Int]){let t=a[2]-2*a[0];print(t*t<8*a[2]*a[1])}

**[Full Test Suite.](http://swift.sandbox.bluemix.net/#/repl/598e2877cb3d25200b1533ab)**