If you are dating couples, it's not cheating because everyone is aware and consenting to the arrangement. The normal (monogamous) rules don't apply.
Also, you're agreeing to spend time with both partners. If you're only interested in one of them and they've asked you to be their third, you should know right now that it's probably not going to work out.
So, provided this is something you want to do and you're attracted to both partners, experience has taught me to follow these guidelines for maximum fun and minimum mess.
1. Start with a long conversation.
Apologies if you're the "a little more action" type, but while a casual threesome can take place as soon as everyone's consenting, a continuing relationship needs a lot of careful thought. You should always talk things out in advance, because if this is all going to end in disaster for one or more of you, it'll probably be obvious from that first conversation.
Make sure you're all compatible and equally enthusiastic. An unconventional setup brings its own challenges and complications, so this is an extremely important step.
2. Be aware of your emotional limits.
Are you also polyamorous? Can you happily fall in love with more than one person? Do you want to? If this couple is keen to get emotionally intimate, you need to know if that's something you can handle.
And what if they want to see people other than you? Will you feel OK about that? Sure, sometimes you won't know until you try it, but if the idea of having serious feelings for multiple people causes a panic attack, emphasise that you need to keep this light and fun.
3. Find out exactly what kind of situation you're getting into.
If you're being approached as a "third", the implication is that the couple will remain primarily committed to each other, with you as an ongoing feature of their relationship.
Over time, this can evolve into a triad situation, in which every party is equally committed to each other. But this is rarer and, if you're just starting out with this couple, it's unlikely to be what they're proposing.
4. Follow their rules...
One you've all decided that you're in, the couple are likely to have a few rules about the way they want their relationship with you to work. If they don't, ask them to make some – it's the only way to be sure no one's overstepping boundaries.
Do they want to keep the relationship on the down low? Do they want it to be mostly about sex with very little "cooking pasta together" domestic bliss? Do they never want to travel to your house? Find out! And if you agree to these rules, FOLLOW THEM.
5. Be generous and open-minded.
Once the business side of things is over and the ~business~ side can begin, make sure to be a giving and thoughtful lover. Inviting someone into your bed is an act of trust, so do your best to honour that trust. One of the beautiful things about being close to an existing couple is that they will have established patterns; integrate yourself into them as organically as you can.
6. Make some rules of your own.
There's no point going into this if you're not going to enjoy it. Figure out what you'll want and need from them and be upfront about it. Hate sleeping over and doing the breakfast thing? Let them know. Need a lot of affirmation and text-y affection? Speak now or get quietly sad, really fast.
Whether you're looking for casual fun or actual inclusion in their lives, you need to establish your expectations early.
7. Know what you will and won't do sexually.
One of the best things about any relationship is experimentation and having your horizons expanded. So there's a chance this couple will teach you some new, fun tricks. But if there are certain items that are permanently off your menu, let them know.
Similarly, if there's a fantasy of yours that can only be fulfilled with two people present, now's your chance to make that happen (well, ask politely for that to happen).