#[Swift 3], 55 bytes

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

    func f(a:Int,b:Int,c:Int){let t=c-2*a;print(t*t<8*c*b)}

**[Full Test Suite.](http://swift.sandbox.bluemix.net/#/repl/598e29d7cb3d25200b1533ad)**

#[Swift 3], 57 bytes

    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)**

  [Swift 3]:  https://www.swift.org