Hi Candy Man,
There could be several different things occurring here and you need to work through them before deciding that a threesome is a good. Threesomes with friends are not always the good idea and it can have disastrous consequences. You starting point must be speaking with your current girlfriend about this and as a part of your discussion needs to you observation about her not minding the flirtations. It could be that she is fine with it or it could be that she is not sure how to address it. By talking talking to her it will give her a chance to air her concern. Also you need to talk to her about the sexual tension you feel that exists. It could be that your girlfriend notices and is interested in exploring it. Alternative it could be that the tension you are feeling is between the other girl and you that is due to her trying to take you away from your current girlfriend.
Provided you are able to get your girlfriend to agree to the threesome my feeling you would need to speak to her about boundaries and agree to them. From there you will need to bring the other girl into the discussion and she is also open to the idea then feeling here is there a few things that need to be discussed. This would include topics such has how does your friendship return, determining if there is any jealousy between the two girls, determining if there are feelings of competition between the two girls, limits to the activity, and determining if the other girl has feelings for either of you. Once you have these discussion the three of you will need to decide how to mitigate issues so that they do not become problematic.
Moving on before answering your question about is it a bad I will answer your question regarding the worst case scenario. Before answering that question I will assume that you understand the risks in regard to pregnancy, STDS, negative influence of alcohol, and the negative influence drugs including cannabis can play in a threesome situation. Outside of these my feeling this type of situation raises three worst cases scenarios. Since the potential other girl is a close friend my feeling is loss of her friendship is a possibility. Another issue is her becoming dependent on the two of you. By dependent she is always wanting to do things together and the relationship begins to develop into a quasi menage de trios relationship due to relationship boundaries becoming blurred. Third issue will be jealousy / competition between the two girls. Your posting suggests to me that there may be some underlying issues but what they are, I cannot exactly say. If jealousy or competition does exist then it is my feeling that this could be an issue that causes your relationship to end. Fourth issue is that even if jealousy or competition does not exist your relationship is at risk due to the fact it could bring up other underlying issues in your relationship.
Finally is this a bad idea? This question is something you need to answer for yourself based on your discussions and knowledge of the two girls. Also you need to define by what you mean by bad idea. Meaning, what constitutes it being a bad idea? What factors go into making it a bad idea?
The key to answering your question lies in a few things. One will be communication which includes the amount, quality, and if everyone follows the boundaries. Another key will be the relationship between the two girl, how they perceive each other, and if they can work through any issues that may come up. This goes a step further to include how well the three of you are able to operate as a cohesive unit to work through issues and communicate. Third will be how the two of you arrive at your decision to have a threesome and if it is an egalitarian decision. Final key will be your relationship with your girlfriend. If it is stable and there is good communication then it gives you a foundation to build upon for having a successful threesome.