https://play.golang.org/p/_TeaX_DkAy9

combinations.go

package main

import (
	"fmt"
	"strings"
)

func Combinations(n int, combs []int) (next func() bool) {
	k := len(combs)
	if k == 0 || n < k {
		return func() bool { return false }
	}
	init := true
	return func() bool {
		if init {
			for i := range combs {
				combs[i] = i
			}
			init = false
			return true
		}
		var (
			i int
		)

		// Combination (n-k, n-k+1, ..., n) reached
		// No more combinations can be generated
		if combs[0] == n-k {
			return false
		}

		for i = k - 1; i >= 0; i-- {
			combs[i]++
			if combs[i] < n-k+1+i {
				break
			}
		}

		// combs now looks like (..., x, n, n, n, ..., n).
		// Turn it into (..., x, x + 1, x + 2, ...)
		for i = i + 1; i < k; i++ {
			combs[i] = combs[i-1] + 1
		}

		return true
	}
}

func main() {
	ss := strings.Split("a,b,c,d,e,f,g", ",")
	combs := make([]int, 3)

	for next := Combinations(len(ss), combs); next(); {
		fmt.Println(ss[combs[0]], ss[combs[1]], ss[combs[2]])
	}

}