main.swift

import Foundation

enum Coin: Int, CustomStringConvertible {
    case Tails, Heads
    
    var description: String {
        if self == .Tails {
            return "T"
        } else {
            return "H"
        }
    }
    
    init?(string: String) {
        if string.uppercaseString == "T" {
            self.init(rawValue: 0)
        } else if string.uppercaseString == "H" {
            self.init(rawValue: 1)
        } else {
            return nil
        }
    }
    
    static func flip() -> Coin {
        return Coin(rawValue: Int(arc4random_uniform(2)))!
    }
}

let input = NSFileHandle.fileHandleWithStandardInput()

func checkQuit() -> Bool {
    print("Play again? ")
    let data = input.availableData
    let str = NSString(data: data, encoding: NSUTF8StringEncoding) as! String
    
    return String(str.lowercaseString.characters.dropLast()) == "y"
}

func getComputerSequence(rand: Bool) -> [Coin] {
    let seq: [Coin]
    
    if !rand {
        seq = [human[1] == .Heads ? .Tails : .Heads] + human[0...1]
    } else {
        seq = [Coin.flip(), Coin.flip(), Coin.flip()]
    }
    
    print("Computer's sequence is: ", seq[0], seq[1], seq[2])
    return seq
}

func getHumanSequence() -> [Coin] {
    var ret = [Coin]()

    print("Enter sequence of three: ")
    let data = input.availableData
    let str = NSString(data: data, encoding: NSUTF8StringEncoding) as! String
    let seq = str.characters.dropLast().map {Coin(string: String($0))}
    
    for case let coin? in seq {
        ret.append(coin)
    }
    
    if ret.count != 3 {
        print("You can only pick three")
        exit(0)
    }
    
    return ret
}

var human = [Coin]()
var computer = [Coin]()
var win = false
var play = true

while play {
    win = false
    switch Coin.flip() {
    case .Heads:
        human = getHumanSequence()
        computer = getComputerSequence(false)
    case .Tails:
        computer = getComputerSequence(true)
        human = getHumanSequence()
    }
    
    while !win {
        let seq = [Coin.flip(), Coin.flip(), Coin.flip()]
        print(seq[0], seq[1], seq[2], separator: "", terminator: "")

        if seq == human {
            print("\nYou win!")
            win = true
        } else if seq == computer {
            print("\nComputer wins!")
            win = true
        }
    }
    
    play = checkQuit()
}

it seems that you are not licensing your code. Have you ever heard of the GPLv3 License ? GPL is the only license that truly respects users freedom. I urge you to read http://www.gnu.org/licenses/gpl-3.0.en.html