you see, here's a critical difference between what women and men think cheating is. for you, having sex with 2 girls or a couple is kinda not a cheating cuz you don't put any emotions to that.. i think that your GF would still consider it as cheating. As much as for me real cheating would be "moral" cheating, i still wouldn't understand it if my husbund or BF would go do that..
If you are asking such a question , you arent ready to do it. Trust your gut , your first impressions are usually right. .........threesomes arent that great...........