An implementation of zip that combines the elements of three sequences.

Zip3Sequence.swift

import Foundation

struct Zip3Sequence<Sequence1, Sequence2, Sequence3>: Sequence
where Sequence1: Sequence, Sequence2: Sequence, Sequence3: Sequence {
    typealias Element = Iterator.Element
    typealias Iterator = Zip3Iterator<Sequence1.Iterator, Sequence2.Iterator, Sequence3.Iterator>

    private var s1: Sequence1
    private var s2: Sequence2
    private var s3: Sequence3

    init(_ s1: Sequence1, _ s2: Sequence2, _ s3: Sequence3) {
        self.s1 = s1
        self.s2 = s2
        self.s3 = s3
    }

    func makeIterator() -> Iterator {
        return Zip3Iterator(s1.makeIterator(), s2.makeIterator(), s3.makeIterator())
    }
}

struct Zip3Iterator<Iterator1, Iterator2, Iterator3>: IteratorProtocol
where Iterator1: IteratorProtocol, Iterator2: IteratorProtocol, Iterator3: IteratorProtocol {
    typealias Element = (Iterator1.Element, Iterator2.Element, Iterator3.Element)

    private var i1: Iterator1
    private var i2: Iterator2
    private var i3: Iterator3

    init(_ i1: Iterator1, _ i2: Iterator2, _ i3: Iterator3) {
        self.i1 = i1
        self.i2 = i2
        self.i3 = i3
    }

    mutating func next() -> Element? {
        guard let v1 = i1.next(),
              let v2 = i2.next(),
              let v3 = i3.next() else { return nil }
        return (v1, v2, v3)
    }
}

func zip3<Sequence1, Sequence2, Sequence3>(_ s1: Sequence1, _ s2: Sequence2, _ s3: Sequence3)
    -> Zip3Sequence<Sequence1, Sequence2, Sequence3>
    where Sequence1: Sequence, Sequence2: Sequence, Sequence3: Sequence {
        return Zip3Sequence(s1, s2, s3)
}