Thrupple advice? Vee relationship.

I’m in a relationship currently ( me F 27 him M 27) and we’ve been together for what feels like forever (6 years) been engaged for some time (3 years) and are as solid as you could imagine. There’s a woman who I’ve considered my best friend for almost as long. I’m so unbelievably attracted to her. We kissed once and I’ve never stopped thinking about it. She expressed wanting more, I do too and my fiance is perfectly okay with it. He actually says he’s indifferent to it and doesn’t really care. Her and him are cool. They say they’re not attracted to each other at all and part of me believes it but I’m also a very insecure person. It’s hard for me to imagine that either one of them is attracted to me in any way. Firstly I see him as a solid 10 and I see myself somewhere at 6 or so. I see her also as a solid 10. Of course this whole situation is scary and in my head it doesn’t make sense that two (seriously beautiful and amazing) people could both be okay with sharing someone? I know it’s my insecurities talking but come on. I’m Hispanic, our nicknames in the family are literally our insecurities (my nickname is gorda or fatty) so I’ve always known how people see my physical body. I think she wants a real relationship and part of me wants to try but I’m scared. What if I don’t give enough attention to both of them? What if they choose each other over me and leave me all together? She’s also my first kiss as a woman so I’ve never done anything more and that’s intimidating af. It would be considered a “vee relationship” according to what I’ve found relating to what our situation would be. They’d both be with me but they wouldn’t be with each other. Any advice or opinions would be appreciated. TYIA.