I completely agree with what Vixtoria has said here. My family is an extremely happy and healthy triad that has been through MUCH in the past 5 years yet we persevere and remain great friends and happy lovers to this very day. I am the husband and my wives have been together for 10 or so years prior to my being a part of the picture. The three of us have been very close friends for almost 17 years...but even so it was very hard for me to not feel like a side dish when we were all together and to feel like I was intruding on them. It is a fine line that takes much compromise, understanding and love to walk and eventually work through. I would recommend bringing someone into your relationship with the understanding that no matter how careful these feelings will likely crop up at done point or another and they are very hard for that person to bring up- be mindful of this.
"Who am I make such an assumption that I deserve an equal role when I am the one who has just arrived in this relationship?".... Tis a conversation I used to have many times over in the beginning.
Best wishes and I hope that your endeavors to find this new relationship bring you feat happiness.