#[Swift 3], 55 bytes

Note that I declared the variable `t` because the expression would be too complex to be solved in reasonable time otherwise (Swift's fault!).

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

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

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

**or 55 bytes**, exact closure equivalent (I need the last part because it is a complex construct):

    {let t=$2-2*$0;return t*t<8*$2*$1}as(Int,Int,Int)->Bool

**[Test Suite.](http://swift.sandbox.bluemix.net/#/repl/598e2ba1cb3d25200b1533af)**

#[Swift 3], 57 bytes

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

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

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