Would it ruin our relationship if we had a threesome with another girl?

Whether it will ruin your relationship or not, will not be known until after the threesome. That's the bummer part of couples having group sex for the first time.....uncharted territory. If you look at the percentages, most couples do have problems. Someone usually feels left out, jealous or inadequate. For me, threesomes are not worth the risk of ruining a good relationship. But I cannot lie, I have had a few and some were really hot and mind blowing. That is always the temptation. Perhaps you are one of the lucky few couples who are secure enough in your relationship not have issues. I would never assume that, but there are a few couples out there that make threesomes work in their relationships. Talk it over carefully with all parties and if anyone is not 100% into doing it with great confidence they will be OK, don't do it.