In order for ANY relationship to survive something like a threesome, you need 3 key things:
1. Complete truth between ALL involved parties. That means you have to trust each other enough to KNOW that it won't change, or ruin, ANY of the relationships (romantic or friendly).
2. Open, honest, and complete communication. All involved parties need to sit down and discuss when, where, and what will be allowed. Any potential problems will have to be addressed BEFOREHAND, and any unforeseen problems will need to be dealt with as SOON as they arise.
3. Complete willingness. That means no one should need convinced, coerced, or otherwise talked into it. Having questions and concerns is fine (and only natural and to be expected). One party being against it and having the other parties talk them into it, is not ok. It will only lead the unwilling party to feel used, pressured, and resentful, ruining ALL the relationships.
Some guidelines I think should be followed:
1. Make sure all involved parties have recent std test results, and use protection.
2. Ideally, it should be the two people in the romantic relationship approaching the third party with the offer to join them.
3. Use a neutral location, like a hotel. It'll make all parties feel more comfortable, and prevent anyone from feeling like an outsider (because of being in someone else's bed) and prevent potentially negative emotions from being associated with a personal bed or room.
4. Decide beforehand if it's a one-time thing, or if it will be reoccurring, and STICK TO IT!