Right on superrich. You should follow that advice even to the point of making the first session ONLY about your GS's pleasure. Even to the point of you not bonking the WG. Only let your GF have your dick the first time and do a lot of double teaming with the WG on her.
The risk is that you will be tempted to hit up the WG the first time because you'll think it might be your last, but do things right on the first 3-way and your GF will be back for more.