I say you should try it out now instead of later. Going through poly tends to be a big experience that can change many perspectives. Better to go through it sooner than later.
Be careful about triads, I use to focus on them more because they sounded more stable than a V. However, I have reversed my opinion of that. With a triad, you or your girlfriend may fall more or less in love with the person. Or the other person may feel more intensely for one than the other. I would suggest not labeling it and let it progress as it will. If it turns into a triad or V, then let it. Trying to push for s specific relationship configuration seems to add instability.
As for getting use to the idea of a girlfriend being with another guy, I use to have the same issue. I realized I had to trust that my SO would not date another guy who would try to steal her away or feel like he is pissing on my territory. I realized I just wanted my relationship with her to be respected by the other guy. After I realized that, I also realized I could feel compersion for her going out with someone else.
It took awhile, but I think it is worth it to have one less thing that can easily bug you.