The thing with negotiating boundaries is that they are meant to be negotiated often and be living. I don't think its healthy not too, other wise people compromise and rules are set.
Fluidity and giving the process of negotiation life means that her request for threesome sex is just that, a request. One that is time specific. If doesn't mean forever in this way, maybe just for a while because she needs to catch up emotionally. The thing would be to make sure you are clear that you can compromise for a time but eventually you need to negotiate your boundaries in terms of opening that up to time alone. In this way you can give, knowing there will be change and she can breath easy knowing that you are giving her time to just be all three.
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